Introduction to Engineering thermodynamics 2 nd Edition, Sonntag and Borgnakke. Solution manual. Chapter 5

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1 Introduction to Engineering thermodynamics nd Edition, Sonntag and Borgnakke Solution manual Chapter 5 Claus Borgnakke he picture is a false color thermal image of the space shuttle s main engine. he sheet in the lower middle is after a normal shock across which you hae changes in, and density. Courtesy of NASA.

2 CONEN SUBSECION ROB NO. Concept roblems -8 Kinetic and potential energy 9-5 roperties (u,h) from general tables 6-5 Energy equation: simple process 6-5 Energy eqaution: multistep process Energy equation: solids and liquids roperties (u, h, C, Cp), ideal gas Energy equation: ideal gas Energy equation: polytropic process Energy equation in rate form 0-4

3 Concept roblems 5. What is cal in SI units and what is the name gien to N-m? Look in the conersion factor table A. under energy: cal (Int.) = J = Nm = kg m /s his was historically defined as the heat transfer needed to bring g of liquid water from 4.5 o C to 5.5 o C, notice the alue of the heat capacity of water in able A.4 N-m = J or Force times displacement = energy = Joule

4 5. Why do we write E or E E whereas we write Q and W? E or E E is the change from state to state and depends only on states and not upon the process between and. Q and W are amounts of energy transferred during the process between and and depend on the process path.

5 5.3 When you windup a spring in a toy or stretch a rubber band what happens in terms of work, energy, and heat transfer? Later, when they are released, what happens then? In both processes work is put into the deice and the energy is stored as potential energy. If the spring or rubber is inelastic some of the work input goes into internal energy (it becomes warmer or permanently deformed) and not into its potential energy. Being warmer than the ambient air it cools slowly to ambient temperature. When the spring or rubber band is released the potential energy is transferred back into work gien to the system connected to the end of the spring or rubber band. If nothing is connected the energy goes into kinetic energy and the motion is then dampened as the energy is transformed into internal energy.

6 5.4 CV A is the mass inside a piston-cylinder, CV B is that plus the piston outside, which is the standard atmosphere. Write the energy equation and work term for the two CVs assuming we hae a non-zero Q between state and state. o m p m A g CV A: E E = m A (e e ) = m A (u u ) = Q WA WA = dv = (V V ) CV B: E E = m A (e e ) + m pist (e e ) = m A (u u ) + m pist (gz gz ) = Q WB WB = o dv = o (V V ) Notice how the inside CV A is = o + m pist g /A cyl i.e. the first work term is larger than the second. he difference between the work terms is exactly equal to the potential energy of the piston sitting on the left hand side in the CV B energy Eq. he two equations are mathematically identical. WA = (V V ) = [ o + m pist g /A cyl ] (V V ) = WB + m pist g(v V )/A cyl = WB + m pist g(z Z )

7 5.5 Saturated water apor has a maximum for u and h at around 35 o C. Is it similar for other substances? Look at the arious substances listed in appendix B. Eeryone has a maximum u and h somewhere along the saturated apor line at different for each substance. his means the constant u and h cures are different from the constant cures and some of them cross oer the saturated apor line twice, see sketch below. Constant h lines are similar to the constant u line shown. C.. u = C C.. = C u = C Notice the constant u(h) line becomes parallel to the constant lines in the superheated apor region for low where it is an ideal gas. In the - diagram the constant u (h) line becomes horizontal.

8 5.6 A rigid tank with pressurized air is used to a) increase the olume of a linear spring loaded piston cylinder (cylindrical geometry) arrangement and b) to blow up a spherical balloon. Assume that in both cases = A + BV with the same A and B. What is the expression for the work term in each situation? he expression is exactly the same, the geometry does not matter as long as we hae the same relation between and V then W = dv = (A + BV) dv = A(V V ) B (V V ) = = 0.5 ( + ) (V V )

9 5.7 You heat a gas 0 K at = C. Which one in able A.5 requires most energy? Why? A constant pressure process in a control mass gies (recall Eq.5.9) q = u u + w = h h C p he one with the highest specific heat is hydrogen, H. he hydrogen has the smallest mass, but the same kinetic energy per mol as other molecules and thus the most energy per unit mass is needed to increase the temperature.

10 5.8 A 500 W electric space heater with a small fan inside heats air by blowing it oer a hot electrical wire. For each control olume: a) wire only b) all the room air and c) total room plus the heater, specify the storage, work and heat transfer terms as + 500W or - 500W or 0 W, neglect any Q. through the room walls or windows. Storage Work Heat transfer Wire 0 W -500 W -500 W Room air 500 W 0 W 500 W ot room 500 W -500 W 500 W

11 Kinetic and otential Energy 5.9 A hydraulic hoist raises a 750 kg car.8 m in an auto repair shop. he hydraulic pump has a constant pressure of 800 ka on its piston. What is the increase in potential energy of the car and how much olume should the pump displace to delier that amount of work? C.V. Car. No change in kinetic or internal energy of the car, neglect hoist mass. E E = E - E = mg (Z Z ) = = J he increase in potential energy is work into car from pump at constant. W = E E = dv = V V = E E = = m3

12 5.0 A 00 kg car is accelerated from 30 to 50 km/h in 5 s. How much work is that? If you continue from 50 to 70 km/h in 5 s; is that the same? he work input is the increase in kinetic energy. E E = (/)m[v - V ] = W = kg [50 30 ] km h = 600 [ ] kg 000 m 3600 s = J = 74. kj he second set of conditions does not become the same E E = (/)m[v - V ] = 600 [ ] kg 000 m 3600 s = kj

13 5. Airplane takeoff from an aircraft carrier is assisted by a steam drien piston/cylinder deice with an aerage pressure of 50 ka. A 7500 kg airplane should be accelerated from zero to a speed of 30 m/s with 30% of the energy coming from the steam piston. Find the needed piston displacement olume. C.V. Airplane. No change in internal or potential energy; only kinetic energy is changed. E E = m (/) (V - 0) = 7500 (/) 30 = J = 7875 kj he work supplied by the piston is 30% of the energy increase. W = dv = ag V = 0.30 (E E ) = = 36.5 kj V = W = 36.5 ag 50 =.89 m3

14 5. Sole roblem 5., but assume the steam pressure in the cylinder starts at 000 ka, dropping linearly with olume to reach 00 ka at the end of the process. C.V. Airplane. E E = m (/) (V - 0) 000 = (/) 30 = J = 7875 kj W = 0.5(E E ) = = 36.5 kj W = dv = (/)( beg + end ) V 00 W V V = W 36.5 = ag /( ) = 4.9 m3

15 5.3 A 00 kg car accelerates from zero to 00 km/h oer a distance of 400 m. he road at the end of the 400 m is at 0 m higher eleation. What is the total increase in the car kinetic and potential energy? KE = ½ m (V - V ) V = 00 km/h = = 7.78 m/s m/s KE = ½ 00 kg ( ) (m/s) = J = 463 kj E = mg(z Z ) = 00 kg m/s ( 0-0 ) m = 7684 J = 7.7 kj

16 5.4 A 5 kg piston is aboe a gas in a long ertical cylinder. Now the piston is released from rest and accelerates up in the cylinder reaching the end 5 m higher at a elocity of 5 m/s. he gas pressure drops during the process so the aerage is 600 ka with an outside atmosphere at 00 ka. Neglect the change in gas kinetic and potential energy, and find the needed change in the gas olume. C.V. iston (E E ) IS. = m(u u ) + m[(/)v 0] + mg (h 0) = (/) = = J = kj Energy equation for the piston is: E E = W gas - W atm = ag V gas o V gas (remark V atm = V gas so the two work terms are of opposite sign) V gas = 9.038/(600 00) = 0.08 m 3 V g o H ag V

17 5.5 A mass of 5 kg is tied to an elastic cord, 5 m long, and dropped from a tall bridge. Assume the cord, once straight, acts as a spring with k = 00 N/m. Find the elocity of the mass when the cord is straight (5 m down). At what leel does the mass come to rest after bouncing up and down? Let us assume we can neglect the cord mass and motion. : V = 0, Z = 0 : V, Z = -5m 3: V 3 = 0, Z 3 = -L, F up = mg = k s L : ½ mv + mg Z = ½ V + mgz Diide by mass and left hand side is zero so ½ V + g Z = 0 V = (-g Z ) / = [ m/s (-5) m] / = 9.9 m/s State 3: m is at rest so F up = F down k s L = mg L = mg k = s 00 kg ms - Nm - = 0.49 m L = L o + L = = 5.49 m BRIDGE V m So: Z = -L = m

18 roperties (u, h) from General ables

19 5.6 Find the missing properties of,,, u, h and x if applicable and plot the location of the three states as points in the - and the - diagrams a. Water at 5000 ka, u = 800 kj/kg b. Water at 5000 ka, = 0.06 m 3 /kg c. R-34a at 35 o C, = 0.0 m 3 /kg a) Look in able B.. at 5000 ka u < u f = => compressed liquid able B..4: between 80 o C and 00 o C = 80 + (00-80) = * = 89. C = ( ) = b) Look in able B.. at 5000 ka > g = => superheated apor able B..3: between 400 o C and 450 o C. = *( )/( ) = * = o C h = *( ) = c) B.5.: f < < g => -phase, = sat = ka, x = ( - f ) / fg = ( )/0.04 = 0.4 u = u f + x u fg = *48.68 = kj/kg C.. C.. States shown are placed relatie to the two-phase region, not to each other. a c b a c b = const.

20 5.7 Find the phase and the missing properties of,,, u and x a. Water at 5000 ka, u = 3000 kj/kg b. Ammonia at 50 o C, = m 3 /kg c. Ammonia at 8 o C, 00 ka d. R-34a at 0 o C, u = 350 kj/kg a) Check in able B.. at 5000 ka: u > u g = 597 kj/kg Goto B..3 it is found ery close to 450 o C, x = undefined, = m 3 /kg b) able B.. at 50 o C: > g = m 3 /kg, so superheated apor able B..: close to 600 ka, u = kj/kg, x = undefined c) able B.. between 5 and 30 o C: We see > sat = 67 ka (30 o C) We conclude compressed liquid without any interpolation. = f = u = u f = ( ) = m 3 /kg ( ) = 30.9 kj/kg d) able B.5. at 0 o C: 7.03 = u f < u < u g = kj/kg so two-phase x = u - u f u = fg 6.6 = , = sat = 57.8 ka = f + x fg = x = m 3 /kg all states are relatie to the two-phase region, not to each other c C.. d a, b C.. c d = const. a b

21 5.8 Find the missing properties. a. H O = 50 C, = 0.0 m 3 /kg =? u =? b. N = 0 K, = 0.8 Ma x =? h =? c. H O = C, = 00 ka u =? =? d. R-34a = 00 ka, = 0. m 3 /kg u =? =? a) able B.. at 50 C: f < < g = sat = 3973 ka x = ( - f )/ fg = ( )/ = u = u f + x u fg = = kj/kg b) able B.6. is lower than sat so it is super heated apor => x = undefined and we find the state in able B.6. able B.6.: h = 4.0 kj/kg c) able B.. : < triple point => B..5: > sat so compressed solid u u i = kj/kg i = m 3 /kg approximate compressed solid with saturated solid properties at same. d) able B.5. > g superheated apor => able B.5.. ~ 3.5 C = 30 + (40 30) ( )/( ) u = ( ) = kj/kg S c L a C.. b V d C.. a d b C.. a b = C d c c

22 5.9 wo kg water at 0 o C with a quality of 5% has its temperature raised 0 o C in a constant olume process. What are the new quality and specific internal energy? State from able B.. at 0 o C = f + x fg = = m 3 /kg State has same at 40 o C also from able B.. x = - f = fg = u = u f + x u fg = = kj/kg C.. C C 0 C 40 0

23 5.0 wo kg water at 00 ka with a quality of 5% has its temperature raised 0 o C in a constant pressure process. What is the change in enthalpy? State from able B.. at 00 ka h = h f + x h fg = = 055. kj/kg State has same from able B.. at 00 ka = sat + 0 = = 40.3 o C so state is superheated apor (x = undefined) from able B..3 0 h = ( ) = kj/kg h h = = 693. kj/kg C.. C.. 00 ka C 0. C 40 0

24 5. Saturated liquid water at 0 o C is compressed to a higher pressure with constant temperature. Find the changes in u and h from the initial state when the final pressure is a) 500 ka, b) 000 ka, c) ka State is located in able B.. and the states a-c are from able B..4 State u [kj/kg] h [kj/kg] u = u - u h = h - h () a b c For these states u stays nearly constant, dropping slightly as goes up. h aries with changes. c b a = 0 o C c,b,a, S c b a L C.. V cb

25 5. Find the missing properties and gie the phase of the ammonia, NH 3. a. = 65 o C, = 600 ka u =? =? b. = 0 o C, = 00 ka u =? =? x =? c. = 50 o C, = 0.85 m 3 /kg u =? =? x =? a) able B.. < sat => superheated apor able B..: = = m 3 /kg u = = 435 kj/kg b) able B..: < sat => x = undefined, superheated apor, from B..: =.453 m 3 /kg ; u = kj/kg c) Sup. ap. ( > g ) able B... = 00 ka, x = undefined u = 383 kj/kg States shown are placed relatie to the two-phase region, not to each other. C.. c a b C.. c 00 ka 600 ka a b

26 5.3 Find the phase and missing properties of,,, u, and x. a. Water at 5000 ka, u = 000 kj/kg (able B. reference) b. R-34a at 0 o C, u = 300 kj/kg c. Nitrogen at 50 K, 00 ka Show also the three states as labeled dots in a - diagram with correct position relatie to the two-phase region. a) Compressed liquid: B..4 interpolate between 0 o C and 40 o C. = 33.3 o C, = m 3 /kg, x = undefined b) able B.5.: u < u g => two-phase liquid and apor x = (u - u f )/u fg = ( )/6.6 = = 0.45 = * = m 3 /kg c) able B.6.: > sat (00 ka) so superheated apor in able B.6. x = undefined = 0.5( ) = m 3 /kg, u = 0.5( ) = 84.7 kj/kg C.. C.. States shown are placed relatie to the two-phase region, not to each other. a b c a b = const. c

27 5.4 Find the missing properties and gie the phase of the substance a. H O = 0 C, = 0.5 m 3 /kg u =? =? x =? b. H O = 00 C, = 0 Ma u =? x =? =? c. N = 00 K, = 00 ka =? u =? d. NH 3 = 00 C, = 0. m 3 /kg =? x =? e. N = 00 K, x = 0.75 =? u =? a) able B..: f < < g => L+V mixture, = 98.5 ka, x = ( )/ = 0.56, u = = kj/kg b) able B..4: compressed liquid, = m 3 /kg, u = 46. kj/kg c) able B.6.: 00 K, 00 ka = m 3 /kg ; u = kj/kg d) able B..: > g => superheated apor, x = undefined B..: = = 694 ka e) able B.6.: 00 K, x = 0.75 = = m 3 /kg u = = 8.8 kj/kg States shown are placed relatie to the two-phase region, not to each other. b C.. a e d c b C.. a e c > = const. d

28 5.5 Find the missing properties among (,,, u, h and x if applicable) and gie the phase of the substance and indicate the states relatie to the two-phase region in both a - and a - diagram. a. R- = 500 ka, h = 30 kj/kg b. R- = 0 o C, u = 00 kj/kg c. R-34a = 40 o C, h = 400 kj/kg a) able B.3.: h > h g = > superheated apor, look in section 500 ka and interpolate = C, = m 3 /kg, u = kj/kg b) able B.4.: u < u g => L+V mixture, = ka x = u - u f u = fg = 0.887, = = m 3 /kg, h = = 9.7 kj/kg c) able B.5.: h < h g => two-phase L + V, look in B.5. at 40 C: x = h - h f h = fg 63.3 = = sat = 07 ka, = = m 3 /kg u = = 38. kj/kg C.. C.. States shown are placed relatie to the two-phase region, not to each other. b, c a b, c = C a

29 Energy Equation: Simple rocess

30 5.6 A 00-L rigid tank contains nitrogen (N ) at 900 K, 3 Ma. he tank is now cooled to 00 K. What are the work and heat transfer for this process? C.V.: Nitrogen in tank. m = m ; Energy Eq.5.: m(u - u ) = Q - W rocess: V = constant, = = V/m => W = 0/ able B.6.: State : = m 3 /kg => m = V/ =. kg u = 69.7 kj/kg State : 00 K, = = V/m, look in able B.6. at 00 K 00 ka: = 0.45 m 3 /kg; u = 7.7 kj/kg 400 ka: = m 3 /kg; u = 69.3 kj/kg so a linear interpolation gies: = ( )/( ) = 34 ka u = ( ) = 70.0 kj/kg, Q = m(u - u ) =. ( ) = kj

31 5.7 A rigid container has 0.75 kg water at 300 o C, 00 ka. he water is now cooled to a final pressure of 300 ka. Find the final temperature, the work and the heat transfer in the process. C.V. Water. Constant mass so this is a control mass Energy Eq.: U - U = m(u - u ) = Q - W rocess eq.: V = constant. (rigid) => W = dv = 0 State : 300 o C, 00 ka => superheated apor able B..3 = 0.38 m 3 /kg, u = 789. kj/kg State : 300 ka and = from able B.. < g two-phase = sat = o C x = - f = fg = u = u f + x u fg = x = 58.5 kj/kg Now the heat transfer is found from the energy equation Q = m(u - u ) + W = m(u - u ) = 0.75 ( ) = -48 kj

32 5.8 I hae kg of liquid water at 0 o C, 00 ka. I now add 0 kj of energy at a constant pressure. How hot does it get if it is heated? How fast does it moe if it is pushed by a constant horizontal force? How high does it go if it is raised straight up? a) Heat at 00 ka. Energy equation: E E = Q W = Q (V V ) = H H = m(h h ) h = h + Q /m = / = kj/kg Back interpolate in able B..: =.5 o C (We could also hae used = Q /mc = 0 / (*4.8) =.4 o C) b) ush at constant. It gains kinetic energy. 0.5 m V = W V = W /m = J/ kg = 4.4 m/s c) Raised in graitational field m g Z = W Z = W /m g = J kg m/s = 09 m Comment: Notice how fast (500 km/h) and how high it should by to hae the same energy as raising the temperature just degrees. I.e. in most applications we can disregard the kinetic and potential energies unless we hae ery high V or Z.

33 5.9 A cylinder fitted with a frictionless piston contains kg of superheated refrigerant R- 34a apor at 350 ka, 00 o C. he cylinder is now cooled so the R-34a remains at constant pressure until it reaches a quality of 75%. Calculate the heat transfer in the process. C.V.: R-34a m = m = m; Energy Eq.5. m(u - u ) = Q - W rocess: = const. W = dv = V = (V - V ) = m( - ) V V State : able B.5. State : able B.5. h = ( )/ = 490 kj/kg h = = 35.7 kj/kg (350.9 ka) Q = m(u - u ) + W = m(u - u ) + m( - ) = m(h - h ) Q = ( ) = kj

34 5.30 Ammonia at 0 C, quality 60% is contained in a rigid 00-L tank. he tank and ammonia is now heated to a final pressure of Ma. Determine the heat transfer for the process. C.V.: NH 3 V Continuity Eq.: m = m = m ; Energy Eq.5.: m(u - u ) = Q - W rocess: Constant olume = & W = 0 State : able B.. two-phase state. = x = m 3 /kg u = = kj/kg m = V/ = 0./0.746 =.48 kg State :, = superheated apor able B.. 00 C, u kj/kg So sole for heat transfer in the energy equation Q = m(u - u ) =.48( ) = kj

35 5.3 A piston/cylinder contains kg water at 0 o C with olume 0. m 3. By mistake someone locks the piston preenting it from moing while we heat the water to saturated apor. Find the final temperature and the amount of heat transfer in the process. C.V. Water. his is a control mass Energy Eq.: m (u u ) = Q W rocess : V = constant W = 0 State :, = V /m = 0. m 3 /kg > f so two-phase x = - f = fg = u = u f + x u fg = x = kj/kg State : = = 0. & x = From the energy equation found in able B.. between 0 C and 5 C = = =.3 C u = ( ) = 600. kj/kg Q = m(u u ) = ( ) = 5.3 kj V V

36 5.3 A test cylinder with constant olume of 0. L contains water at the critical point. It now cools down to room temperature of 0 C. Calculate the heat transfer from the water. C.V.: Water m = m = m ; Energy Eq.5.: m(u - u ) = Q - W rocess: Constant olume = roperties from able B.. State : = c = m 3 /kg, u = 09.6 kj/kg m = V/ = kg State :, = = x Constant olume => x = , u = x 39 = kj/kg W = 0/ Q = m(u - u ) = 0.037( ) = -6.7 kj

37 5.33 Find the heat transfer for the process in roblem 4.30 C.. ake as CV the.5 kg of water. m = m = m ; Energy Eq.5. m(u u ) = Q W rocess Eq.: = A + BV (linearly in V) State : (, ) => = m 3 /kg, u = kj/kg State : (, ) => = m 3 /kg, u = 88. kj/kg From process eq.: W = dv = area = m ( + )( ) =.5 ( )( ) = 9.4 kj From energy eq.: Q = m(u u ) + W =.5( ) 9.4 = 65.4 kj

38 5.34 A 0-L rigid tank contains R- at 0 C, 80% quality. A 0-A electric current (from a 6-V battery) is passed through a resistor inside the tank for 0 min, after which the R- temperature is 40 C. What was the heat transfer to or from the tank during this process? C.V. R- in tank. Control mass at constant V. Continuity Eq.: m = m = m ; Energy Eq.: m(u - u ) = Q - W rocess: Constant V = => no boundary work, but electrical work State from table B.4. = = m 3 /kg u = = 84.9 kj/kg m = V/ = 0.00/0.054 = kg State : able B.4. at 40 C and = = m 3 /kg V => superheated apor, so use linear interpolation to get = ( )/( ) = 535 ka, u = ( ) = 50. kj/kg W elec = power t = Amp olts t = Q = m(u u ) + W = ( ) 36 = 3.5 kj = 36 kj

39 5.35 A constant pressure piston/cylinder assembly contains 0. kg water as saturated apor at 400 ka. It is now cooled so the water occupies half the original olume. Find the heat transfer in the process. C.V. Water. his is a control mass. Energy Eq.5.: m(u u ) = Q W rocess: = constant => W = m( ) So sole for the heat transfer: Q = m(u - u ) + W = m(u - u ) + m( - ) = m(h - h ) State : able B.. = m 3 /kg; h = kj/kg State : = / = 0.33 = f + x fg from able B.. x = ( f ) / fg = ( ) / = h = h f + x h fg = = kj/kg Now the heat transfer becomes Q = 0. ( ) = 3.9 kj C.. C = 400 ka cb

40 5.36 wo kg water at 0 o C with a quality of 5% has its temperature raised 0 o C in a constant olume process as in Fig What are the heat transfer and work in the process? C.V. Water. his is a control mass Energy Eq.: m (u u ) = Q W rocess : V = constant W = dv = 0 State :, x from able B.. = f + x fg = = m 3 /kg u = u f + x u fg = = kj/kg State :, = < g = m 3 /kg so two-phase From the energy equation x = - f = fg = u = u f + x u fg = x 96.3 = kj/kg Q = m(u u ) = ( ) = kj C.. C C 0 C 40 0

41 5.37 A 5 kg mass moes with 5 m/s. Now a brake system brings the mass to a complete stop with a constant deceleration oer a period of 5 seconds. he brake energy is absorbed by 0.5 kg water initially at 0 o C, 00 ka. Assume the mass is at constant and. Find the energy the brake remoes from the mass and the temperature increase of the water, assuming = C. C.V. he mass in motion. E - E = E = 0.5 mv = /000 = 7.85 kj C.V. he mass of water. m(u - u ) H O = E = 7.85 kj => u - u = 7.85 / 0.5 = 5.63 kj/kg u = u = = kj/kg Assume u = u f then from able B..: 3.7 o C, = 3.7 o C We could hae used u - u = C with C from able A.4: C = 4.8 kj/kg K giing = 5.63/4.8 = 3.7 o C.

42 5.38 An insulated cylinder fitted with a piston contains R- at 5 C with a quality of 90% and a olume of 45 L. he piston is allowed to moe, and the R- expands until it exists as saturated apor. During this process the R- does 7.0 kj of work against the piston. Determine the final temperature, assuming the process is adiabatic. ake CV as the R-. Continuity Eq.: m = m = m ; Energy Eq.5.: m(u u ) = Q - W State : (, x) abel B.3. => = = m 3 /kg m = V / = 0.045/ =.856 kg u = = kj/kg State : (x =,?) We need one property information. Apply now the energy equation with known work and adiabatic so Q = 0/ = m(u - u ) + W =.856 (u ) => u = kj/kg = u g at able B.3. gies u g at different temperatures: -5 C

43 5.39 Find the heat transfer for the process in roblem 4.34 ake CV as the Ammonia, constant mass. Continuity Eq.: m = m = m ; Energy Eq.: m(u u ) = Q W rocess: = A + BV (linear in V) State : Superheated apor = m 3 /kg, u = 36.7 kj/kg State : Superheated apor = m 3 /kg, u = 54.0 kj/kg Work is done while piston moes at increasing pressure, so we get W = dv = area = ag (V V ) = ( + )m( ) = ½( ) ka 0.5 kg ( ) m 3 /kg =.683 kj Heat transfer is found from the energy equation Q = m(u u ) + W = 0.5 kg ( ) kj/kg kj = = 4.3 kj

44 5.40 A water-filled reactor with olume of m3 is at 0 Ma, 360 C and placed inside a containment room as shown in Fig he room is well insulated and initially eacuated. Due to a failure, the reactor ruptures and the water fills the containment room. Find the minimum room olume so the final pressure does not exceed 00 ka. C.V.: Containment room and reactor. Mass: m = m = V reactor / = / = kg Energy: m(u - u ) = Q - W = 0-0 = 0 State : able B..4 = m 3 /kg, u = 70.8 kj/kg Energy equation then gies u = u = 70.8 kj/kg State : = 00 ka, u < u g => wo-phase able B.. x = (u - u f )/ u fg = ( )/05.0 = = = m 3 /kg V = m = = 87.7 m ka u = const L C.. 00 ka

45 5.4 A piston/cylinder arrangement has the piston loaded with outside atmospheric pressure and the piston mass to a pressure of 50 ka, shown in Fig It contains water at C, which is then heated until the water becomes saturated apor. Find the final temperature and specific work and heat transfer for the process. C.V. Water in the piston cylinder. Continuity: m = m, Energy Eq. per unit mass: u - u = q - w rocess: = constant =, => w = d = ( - ) State :, => able B..5 compressed solid, take as saturated solid. = m 3 /kg, u = kj/kg State : x =, = = 50 ka due to process => able B.. = g ( ) =.593 m 3 /kg, =.4 C ; u = 59.7 kj/kg From the process equation w = ( - ) = 50( ) = 73.7 kj/kg From the energy equation q = u - u + w = (-337.6) = 303 kj/kg L C.. C.. C.. S L+V V = C S+V

46 5.4 A piston/cylinder assembly contains kg of liquid water at 0 o C and 300 ka. here is a linear spring mounted on the piston such that when the water is heated the pressure reaches Ma with a olume of 0. m 3. Find the final temperature and the heat transfer in the process. ake CV as the water. m = m = m ; m(u u ) = Q - W State : Compressed liquid, take saturated liquid at same temperature. = f (0) = m 3 /kg, u = u f = kj/kg State : = V /m = 0./ = 0. m 3 /kg and = 000 ka => wo phase as < g so = sat = 79.9 C x = ( - f ) / fg = ( )/0.933 = u = u f + x u fg = = kj/kg Work is done while piston moes at linearly arying pressure, so we get W = dv = area = ag (V V ) = 0.5 ( )( ) = kj Heat transfer is found from the energy equation Q = m(u u ) + W = ( ) = 684 kj cb

47 5.43 Find the heat transfer for the process in roblem 4.35 ake CV as the water. his is a constant mass: m = m = m ; m(u u ) = Q - W State : Compressed liquid, take saturated liquid at same temperature. B..: = f (0) = m 3 /kg, u = u f (0) = kj/kg State : = V /m = 0./ = 0. m 3 /kg and = 3000 ka from B..3 => Superheated apor Interpolate: = 404 o C, u = kj/kg Work is done while piston moes at linearly arying pressure, so we get: W = dv = area = ag (V V ) = ( + )(V - V ) = 0.5 ( )( ) = kj Heat transfer is found from the energy equation Q = m(u u ) + W = ( ) = 309. kj C.. C ka

48 5.44 A closed steel bottle contains ammonia at 0 C, x = 0% and the olume is 0.05 m 3. It has a safety ale that opens at a pressure of.4 Ma. By accident, the bottle is heated until the safety ale opens. Find the temperature and heat transfer when the ale first opens. C.V.: NH 3 : m = m = m ; Energy Eq.5.: m(u - u ) = Q - W rocess: constant olume process W = 0 State : (, x) able B.. = = 0.59 m 3 /kg => m = V/ = 0.05/0.59 = kg u = = kj/kg V State :, = => superheated apor, interpolate in able B..: 0 C = ( )/( ), u = 48 + ( ) 0.5 = 50.5 kj/kg Q = m(u - u ) = 0.397( ) = kj

49 5.45 wo kg water at 00 ka with a quality of 5% has its temperature raised 0 o C in a constant pressure process. What are the heat transfer and work in the process? C.V. Water. his is a control mass Energy Eq.: m (u u ) = Q W rocess : = constant W = dv = m ( ) State : wo-phase gien,x so use able B.. = = 0.3 m 3 /kg u = = kj/kg = + 0 = = 40.3 State is superheated apor 0 = ( ) = m3 /kg u = From the process equation we get ( ) = 56.3 kj/kg W = m ( ) = 00 ( ) = 85.3 kj From the energy equation Q = m (u u ) + W = ( ) = = kj V V

50 5.46 wo kilograms of nitrogen at 00 K, x = 0.5 is heated in a constant pressure process to 300 K in a piston/cylinder arrangement. Find the initial and final olumes and the total heat transfer required. ake CV as the nitrogen. Continuity Eq.: m = m = m ; Energy Eq.5.: m(u u ) = Q - W rocess: = constant State : able B.6. W = dv = m( - ) = = m 3 /kg, V = m 3 h = = 7.4 kj/kg State : ( = 779. ka, 300 K) => sup. apor interpolate in able B.6. = ( ) 79./00 = 0.5 m 3 /kg, V = 0.3 m 3 h = ( ) 79./00 = kj/kg Now sole for the heat transfer from the energy equation Q = m(u - u ) + W = m(h - h ) = ( ) = 605 kj V V

51 5.47 wo tanks are connected by a ale and line as shown in Fig he olumes are both m3 with R-34a at 0 C, quality 5% in A and tank B is eacuated. he ale is opened and saturated apor flows from A into B until the pressures become equal. he process occurs slowly enough that all temperatures stay at 0 C during the process. Find the total heat transfer to the R-34a during the process. C.V.: A + B State A: A = = m 3 /kg u A = = 5.35 kj/kg m A = V A / A = kg rocess: Constant temperature and constant total olume. m = m A ; V = V A + V B = m 3 ; = V /m = m 3 /kg W = dv = 0 State :, x = ( )/ = 0.33 u = = kj/kg Q = m u - m A u A - m B u B + W = m (u - u A ) = ( ) = 4603 kj A B

52 5.48 Consider the same system as in the preious problem. Let the ale be opened and transfer enough heat to both tanks so all the liquid disappears. Find the necessary heat transfer. C.V. A + B, so this is a control mass. State A: A = = m 3 /kg u A = = 5.35 kj/kg m A = V A / A = kg rocess: Constant temperature and total olume. m = m A ; V = V A + V B = m 3 ; = V /m = m 3 /kg State : x = 00%, = = ( )/( ) = 57.8 C u = ( ) = kj/kg Q = m u - m A u A - m B u B + W = m (u - u A ) = ( ) = 5 50 kj A B

53 5.49 Superheated refrigerant R-34a at 0 C, 0.5 Ma is cooled in a piston/cylinder arrangement at constant temperature to a final two-phase state with quality of 50%. he refrigerant mass is 5 kg, and during this process 500 kj of heat is remoed. Find the initial and final olumes and the necessary work. C.V. R-34a, this is a control mass. Continuity: m = m = m ; Energy Eq.5.: m(u -u ) = Q - W = W State :, able B.5., = m 3 /kg ; u = kj/kg => V = m = 0. m 3 State :, x able B.5. u = = 308. kj/kg, = = m 3 /kg => V = m = 0.09 m 3 W = m(u - u ) = ( ) = kj

54 5.50 A -L capsule of water at 700 ka, 50 C is placed in a larger insulated and otherwise eacuated essel. he capsule breaks and its contents fill the entire olume. If the final pressure should not exceed 5 ka, what should the essel olume be? C.V. Larger essel. Continuity: m = m = m = V/ = 0.96 kg rocess: expansion with Q = 0/, W = 0/ Energy: m(u - u ) = Q - W = 0/ u = u State : f = m 3 /kg; u uf = kj/kg State :, u x = = = = 0.55 m 3 /kg V = m = = 0.5 m 3 = 5 L ka u = const L C.. 00 ka

55 5.5 A rigid tank is diided into two rooms by a membrane, both containing water, shown in Fig Room A is at 00 ka, = 0.5 m3/kg, VA = m3, and room B contains 3.5 kg at 0.5 Ma, 400 C. he membrane now ruptures and heat transfer takes place so the water comes to a uniform state at 00 C. Find the heat transfer during the process. C.V.: Both rooms A and B in tank. A B Continuity Eq.: m = m A + m B ; Energy Eq.: m u - m A u A - m B u B = Q - W State A: (, ) able B.., m A = V A / A = /0.5 = kg x A = f fg = = u A = u f + x u fg = = kj/kg State B: able B..3, B = 0.673, u B = 963., V B = m B B =.6 m 3 rocess constant total olume: V tot = V A + V B = 3.6 m 3 and W = 0 m = m A + m B = 5.5 kg => = V tot /m = m 3 /kg State :, able B.. two-phase as < g x = f fg = = 0.343, u = u f + x u fg = = kj/kg Heat transfer is from the energy equation Q = m u - m A u A - m B u B = = 74 kj

56 5.5 A 0-m high open cylinder, Acyl = 0. m, contains 0 C water aboe and kg of 0 C water below a 98.5-kg thin insulated floating piston, shown in Fig Assume standard g, o. Now heat is added to the water below the piston so that it expands, pushing the piston up, causing the water on top to spill oer the edge. his process continues until the piston reaches the top of the cylinder. Find the final state of the water below the piston (,, ) and the heat added during the process. C.V. Water below the piston. iston force balance at initial state: F = F = A A = m p g + m B g + 0 A State A,B : Comp. Liq. f = m 3 /kg; u A = kj/kg V A = m A A = 0.00 m 3 ; m tot = V tot / = / = 998 kg mass aboe the piston m B = m tot - m A = 996 kg A = 0 + (m p + m B )g/a = ( ) = 8.5 ka State A : A = 0 + m pg A = 0.8 ka ; A = V tot / m A = 0.5 m 3 /kg x A = ( )/.483 = 0.35 ; = 05 C u A = = 69.5 kj/kg Continuity eq. in A: m A = m A Energy: m A (u - u ) = Q - W rocess: linear in V as m B is linear with V W = dv = ( )( ) cb W = 69.3 kj V Q = m A (u - u ) + W = = kj

57 Energy Equation: Multistep Solution

58 kg of water in a piston cylinder arrangement exists as saturated liquid/apor at 00 ka, with a quality of 50%. It is now heated so the olume triples. he mass of the piston is such that a cylinder pressure of 00 ka will float it, as in Fig Find the final temperature and the heat transfer in the process. ake CV as the water. Continuity Eq.: m = m = m ; Energy Eq.: m(u u ) = Q W rocess: = constant until = lift, then is constant. State : wo-phase so look in able B.. at 00 ka u = = 46.7 kj/kg, = = m 3 /kg State :, lift => = =.545 m 3 /kg ; Interpolate: = 89 C, u = kj/kg => V = m = 5.45 m 3 From the process equation (see -V diagram) we get the work as W = lift (V V ) = 00 0 ( ) = 3390 kj From the energy equation we sole for the heat transfer Q = m(u u ) + W = 0 ( ) = 5 96 kj o cb HO cb V

59 5.54 A ertical cylinder fitted with a piston contains 5 kg of R- at 0 C, shown in Fig Heat is transferred to the system, causing the piston to rise until it reaches a set of stops at which point the olume has doubled. Additional heat is transferred until the temperature inside reaches 50 C, at which point the pressure inside the cylinder is.3 Ma. a. What is the quality at the initial state? b. Calculate the heat transfer for the oerall process. C.V. R-. Control mass goes through process: -> -> 3 As piston floats pressure is constant ( -> ) and the olume is constant for the second part ( -> 3). So we hae: 3 = = State 3: able B.4. (,) 3 = m 3 /kg, u 3 = 48.4 kj/kg o 3 cb R- V So we can then determine state and able B.4.: = = x => x = b) u = = 03.5 kj/kg State : = m 3 /kg, = = 68 ka this is still -phase. We get the work from the process equation (see -V diagram) W 3 = W = dv = (V - V ) = 68 5 ( ) = 34. kj he heat transfer from the energy equation becomes Q 3 = m(u 3 -u ) + W 3 = 5( ) = kj

60 5.55 Find the heat transfer in roblem A piston/cylinder contains kg of liquid water at 0 C and 300 ka. Initially the piston floats, similar to the setup in roblem 4.64, with a maximum enclosed olume of 0.00 m 3 if the piston touches the stops. Now heat is added so a final pressure of 600 ka is reached. Find the final olume and the work in the process. ake CV as the water. roperties from table B. m = m = m ; m(u - u ) = Q - W State : Compressed liq. = f (0) = m 3 /kg, u = u f = kj/kg State : Since > lift then = stop = 0.00 and = 600 ka For the gien : f < < g so -phase = sat = C = 0.00 = x ( ) => x = u = = kj/kg Work is done while piston moes at lift = constant = 300 ka so we get W = dv = m lift ( - ) = 300( ) = 0.99 kj Heat transfer is found from energy equation Q = m(u - u ) + W = ( ) = kj o cb H O V

61 5.56 Refrigerant- is contained in a piston/cylinder arrangement at Ma, 50 C with a massless piston against the stops, at which point V = 0.5 m3. he side aboe the piston is connected by an open ale to an air line at 0 C, 450 ka, shown in Fig he whole setup now cools to the surrounding temperature of 0 C. Find the heat transfer and show the process in a diagram. C.V.: R-. Control mass. Continuity: m = constant, Air line Energy Eq.5.: m(u - u ) = Q - W rocess: F = F = A = air A + F stop if V < V stop F stop = 0/ his is illustrated in the - diagram shown below. State : = m 3 /kg, u = 5. kj/kg R- m = V/ = kg State : and on line compressed liquid, see figure below. f = m 3 /kg V = m 3 ; u = u f = kj/kg W = dv = lift (V - V ) = 450 ( ) = -.0 kj ; Energy eq. Q = ( ) - = kj = Ma Ma = 0 50 ~73 = 450 ka 450 ka.96 0

62 5.57 Find the heat transfer in roblem A piston/cylinder (Fig. 4.58) contains kg of water at 0 C with a olume of 0. m 3. Initially the piston rests on some stops with the top surface open to the atmosphere, o and a mass so a water pressure of 400 ka will lift it. o what temperature should the water be heated to lift the piston? If it is heated to saturated apor find the final temperature, olume and the work, W. C.V. Water. his is a control mass. m = m = m ; m(u - u ) = Q - W o a H O V State : 0 C, = V/m = 0./ = 0. m 3 /kg x = ( )/ = u = = 87.9 kj/kg o find state check on state a: = 400 ka, = = 0. m 3 /kg able B..: f < < g = m 3 /kg State is saturated apor at 400 ka since state a is two-phase. = g = m 3 /kg, V = m = m 3, u = u g = kj/kg ressure is constant as olume increase beyond initial olume. W = dv = (V - V ) = lift (V V ) = 400 ( ) = 45 kj Q = m(u - u ) + W = ( ) + 45 = 60.7 kj

63 5.58 Calculate the heat transfer for the process described in roblem 4.6. A piston cylinder setup similar to roblem 4.4 contains 0. kg saturated liquid and apor water at 00 ka with quality 5%. he mass of the piston is such that a pressure of 500 ka will float it. he water is heated to 300 C. Find the final pressure, olume and the work, W. ake CV as the water: m = m = m Energy Eq.5.: m(u - u ) = Q - W lift a rocess: = constant until = lift o locate state : able B.. = = m 3 /kg cb V u = = kj/kg State a: 500 ka, a = = > g at 500 ka, so state a is superheated apor able B..3 a = 00 C State is 300 C so heating continues after state a to at constant = 500 ka. :, = lift => bl B..3 =0.556 m 3 /kg; u = 80.9 kj/kg From the process, see also area in -V diagram W = lift m( - ) = ( ) = 4.9 kj From the energy equation Q = m(u - u ) + W = 0.( ) = 9.5 kj

64 5.59 A cylinder/piston arrangement contains 5 kg of water at 00 C with x = 0% and the piston, m = 75 kg, resting on some stops, similar to Fig he outside pressure is 00 ka, and the cylinder area is A = 4.5 cm. Heat is now added until the water reaches a saturated apor state. Find the initial olume, final pressure, work, and heat transfer terms and show the diagram. C.V. he 5 kg water. Continuty: m = m = m ; Energy: m(u - u ) = Q - W rocess: V = constant if < lift otherwise = lift see - diagram. 3 = = lift = 0 + m p g / A p = = 400 ka 3 o 43 C o cb cb o 00 C H O State : (,x) able B.. = , V = m = =.677 m 3 u = = kj/kg State 3: (, x = ) able B.. => 3 = >, u 3 = kj/kg Work is seen in the -V diagram (if olume changes then = lift ) W 3 = W 3 = extm( 3 - ) = 400 5( ) = 54. kj Heat transfer is from the energy equation Q 3 = 5 ( ) = 8840 kj

65 Energy Equation: Solids and Liquids

66 5.60 In a sink 5 liters of water at 70 o C is combined with kg aluminum pots, kg of flatware (steel) and kg of glass all put in at 0 o C. What is the final uniform temperature neglecting any heat loss and work? Energy Eq.: U - U = m i(u - u ) i = Q - W = 0 For the water: f = m 3 /kg, V = 5 L = m 3 ; m = V/ = kg For the liquid and the metal masses we will use the specific heats (bl A.3, A.4) so m i(u - u ) i = m ic i ( - ) i = m ic i m ic i i noticing that all masses hae the same but not same initial. m ic i = =.59 kj/k Energy Eq.:.59 = ( ) 0 = = 65. o C

67 5.6 Because a hot water supply must also heat some pipe mass as it is turned on so it does not come out hot right away. Assume 80 o C liquid water at 00 ka is cooled to 45 o C as it heats 5 kg of copper pipe from 0 to 45 o C. How much mass (kg) of water is needed? C.V. Water and copper pipe. No external heat transfer, no work. Energy Eq.5.: U U = U cu + U H O = 0 0 From Eq.5.8 and able A.3: U cu = mc Τ = 5 kg 0.4 From the energy equation m H O = - U cu / u H O m H O = U cu / C H O(- Τ H O) = or using able B.. for water m H O = U cu / ( u - u ) = kj kg K (45 0) K = 57.5 kj =.076 kg =.076 kg Cu pipe Water he real problem inoles a flow and is not analyzed by this simple process.

68 5.6 A copper block of olume L is heat treated at 500 C and now cooled in a 00-L oil bath initially at 0 C, shown in Fig Assuming no heat transfer with the surroundings, what is the final temperature? C.V. Copper block and the oil bath. Also assume no change in olume so the work will be zero. Energy Eq.: U - U = m met (u - u ) met + m oil (u - u ) oil = Q - W = 0 roperties from able A.3 and A.4 m met = Vρ = 0.00 m kg/m 3 = 8.3 kg, m oil = Vρ = 0. m 3 90 kg/m 3 = 8 kg Solid and liquid Eq.5.7: u C, able A.3 and A.4: C met = 0.4 he energy equation for the C.V. becomes kj kg K, C kj oil =.8 kg K m met C met (,met ) + m oil C oil (,oil ) = ( 500) ( 0) = = 0 = 5 C

69 5.63 A house is being designed to use a thick concrete floor mass as thermal storage material for solar energy heating. he concrete is 30 cm thick and the area exposed to the sun during the daytime is 4 m 6 m. It is expected that this mass will undergo an aerage temperature rise of about 3 C during the day. How much energy will be aailable for heating during the nighttime hours? C.V.: Control mass concrete. V = = 7. m 3 Concrete is a solid with some properties listed in able A.3 m = ρv = 00 kg/m 3 7. m 3 = kg Energy Eq.: m(u - u ) = Q - W = Q he aailable heat transfer is the change in U. From Eq.5.8 and C from table A.3 U = m C = kg 0.88 kj kg K 3 K = 488 kj = 4.8 MJ

70 5.64 A kg steel pot contains kg liquid water both at 5 o C. It is now put on the stoe where it is heated to the boiling point of the water. Neglect any air being heated and find the total amount of energy needed. Energy Eq.: U U = Q W : he steel does not change olume and the change for the liquid is minimal, so W 0. State : = sat (atm) = 00 o C bl B.. : u = 6.98 kj/kg, u = 48.9 kj/kg bl A.3 : C st = 0.46 kj/kg K Sole for the heat transfer from the energy equation Q = U U = m st (u u ) st + m HO (u u ) HO = m st C st ( ) + m HO (u u ) HO Q = kg 0.46 = = 395 kj kj kg K (00 5) K + kg ( ) kj/kg

71 5.65 A car with mass 75 kg dries at 60 km/h when the brakes are applied quickly to decrease its speed to 0 km/h. Assume the brake pads are 0.5 kg mass with heat capacity of. kj/kg K and the brake discs/drums are 4.0 kg steel. Further assume both masses are heated uniformly. Find the temperature increase in the brake assembly. C.V. Car. Car loses kinetic energy and brake system gains internal u. No heat transfer (short time) and no work term. m = constant; Energy Eq.5.: E - E = 0-0 = m car (V V ) + m brake(u u ) he brake system mass is two different kinds so split it, also use C from able A.3 since we do not hae a u table for steel or brake pad material. m steel C + m pad C = m car 0.5 (60 0 ) m /s ( ) kj K = 75 kg 0.5 ( ) m /s = J = 57.4 kj => = 65.9 C

72 5.66 A computer CU chip consists of 50 g silicon, 0 g copper, 50 g polyinyl chloride (plastic). It heats from 5 o C to 70 o C as the computer is turned on. How much energy does the heating require? Energy Eq.: U - U = m i(u - u ) i = Q - W For the solid masses we will use the specific heats, able A.3, and they all hae the same temperature so m i(u - u ) i = m ic i ( - ) i = ( ) m ic i m ic i = = kj/k U - U = (70 5) = 5.03 kj

73 5.67 A 5 kg steel tank initially at 0 o C is filled up with 00 kg of milk (assume properties as water) at 30 o C. he milk and the steel come to a uniform temperature of +5 o C in a storage room. How much heat transfer is needed for this process? C.V. Steel + Milk. his is a control mass. Energy Eq.5.: U U = Q W = Q rocess: V = constant, so there is no work W = 0. Use Eq.5.8 and alues from A.3 and A.4 to ealuate changes in u Q = m steel (u - u ) steel + m milk (u - u ) milk = 5 kg kj kg K [5 ( 0)] Κ + 00 kg 4.8 kj kg K (5 30) Κ = = 077 kj

74 5.68 An engine consists of a 00 kg cast iron block with a 0 kg aluminum head, 0 kg steel parts, 5 kg engine oil and 6 kg glycerine (antifreeze). Eerything begins at 5 o C and as the engine starts we want to know how hot it becomes if it absorbs a net of 7000 kj before it reaches a steady uniform temperature. Energy Eq.: U U = Q W rocess: he steel does not change olume and the change for the liquid is minimal, so W 0. So sum oer the arious parts of the left hand side in the energy equation m Fe (u u ) + m Al (u u ) Al + m st (u u ) st + m oil (u u ) oil + m gly (u u ) gly = Q bl A.3 : C Fe = 0.4, C Al = 0.9, C st = 0.46 all units of kj/kg K bl A.4 : C oil =.9, C gly =.4 all units of kj/kg K So now we factor out as u u = C( ) for each term [ m Fe C Fe + m Al C Al + m st C st + m oil C oil + m gly C gly ] ( ) = Q = Q / Σm i C i 7000 = = = 75 K = + 75 = = 80 o C Shaft power Air intake filter Fan Radiator Atm. air Exhaust flow Coolant flow

75 roperties (u, h, C and Cp), Ideal Gas

76 5.69 Use the ideal gas air table A.7 to ealuate the heat capacity C p at 300 K as a slope of the cure h() by h/. How much larger is it at 000 K and 500 K. Solution : From Eq.5.4: C p = dh d = h = h 30 - h =.005 kj/kg K 000K C p = h = h h = 00 =.40 kj/kg K 500K C p = h = h h = 00 =. kj/kg K Notice an increase of 4%, % respectiely. h C p C p 300

77 5.70 We want to find the change in u for carbon dioxide between 600 K and 00 K. a) Find it from a constant C o from table A.5 Solution : b) Find it from a C o ealuated from equation in A.6 at the aerage. c) Find it from the alues of u listed in table A.8 a) u C o = (00 600) = 39.8 kj/kg b) ag = ( ) = 900, θ = 000 = = 0.9 C po = =.086 kj/kg K C o = C po R = =.097 kj/kg K u =.097 (00 600) = 6.8 kj/kg c) u = = kj/kg u u 00 u

78 5.7 Water at 50 C, 400 ka, is brought to 00 C in a constant pressure process. Find the change in the specific internal energy, using a) the steam tables, b) the ideal gas water table A.8, and c) the specific heat from A.5. a) b) State : able B..3 State : able B..3 u = kj/kg Superheated apor u = kj/kg u - u = = kj/kg able A.8 at 43.5 K: u = 59.4 kj/kg able A.8 at K: u = kj/kg u - u = = 88.8 kj/kg c) able A.5 : C o =.4 kj/kgk u - u =.4 (00 50) = kj/kg Notice how the aerage slope from 50 C to 00 C is higher than the one at 5 C ( = C o ) u 00 u Slope at 5 C u

79 5.7 We want to find the increase in temperature of nitrogen gas at 00 K when the specific internal energy is increased with 40 kj/kg. a) Find it from a constant C o from table A.5 Solution : b) Find it from the alues of u listed in table A.8 u = u A.8 C ag C o a) = u / C o = = C b) u = u + u = = 997 kj/kg less than 300 K so linear interpolation. = C o = C ( ) / 00 = 0.95 kj/kg K So the A.5 alue is much lower than the alue at 00 K u u kj/kg