EP145 Introduction to Engineering

Transcription

1 EP145 Intrductin t Engineering Tpic 8 Temperature Related Parameters Department f Engineering Physics University f Gaziantep Dec 2012 Sayfa 1 Intrductin In this chapter we will cnsider sme cncepts related t Temperature and Heat in Engineering. Details can be fund in [1] and [2]. Sayfa 2 1

2 Temperature What is Temperature? It is a physical prperty f matter that expresses the sense f ht, warm r cld. A cnvenient peratinal definitin: temperature is that it is a measure f the average translatinal kinetic energy assciated with the disrdered micrscpic mtin f atms and mlecules. S, the temperature represents the level f mlecular activity f a substance. Air mlecules are mre active at 50 C than they are at 25 C. Sayfa 3 When a high temperature bject is placed in cntact with a lw temperature bject, then energy will flw frm the high temperature bject t the lwer temperature bject, and they will apprach an equilibrium temperature. Sayfa 4 2

3 Temperature Measurement Temperature measurement using mdern scientific thermmeters and temperature scales ges back at least as far as the early 18 th century. The basic unit f temperature in SI is the kelvin (K). Many methds have been develped fr measuring temperature. Glass thermmeter filled with mercury Thermcuples Thermistrs Resistance Temperature Detectr (RTD) Pyrmeter Langmuir prbes (fr electrn temperature f a plasma) Infrared thermmeters Sayfa 5 An image f tw peple in mid-infrared ("thermal") light (false-clr) Sayfa 6 3

4 Much f a persn's energy is radiated away in the frm f infrared light. Sme materials are transparent in the infrared, while paque t visible light. Sayfa 7 Sayfa 8 4

5 The distributin f the csmic micrwave backgrund radiatin acrss the universe as measured by the Wilkinsn Micrwave Anistrpy Prbe. Sayfa 9 Temperature Scales Much f the wrld uses the Celsius scale ( C). The United States uses the Fahrenheit ( F) scale. Celcius has the same incremental scaling as the Kelvin scale (K) used by scientists. T( C) T( K) T ( F) 9 T ( 5 C) 32 There are ther scales. Sayfa 10 5

6 Sayfa 11 Sayfa 12 6

7 Many physical prperties f materials depend n temperature. Such as: Matter phase slid, liquid, gaseus r plasma density viscsity slubility vapr pressure electrical cnductivity Engineers must have a gd understanding f temperature and its rle in design. Sayfa 13 Example 1 The air resistance t yur car s mtin is greater in winter than it is in summer, if the car is mving at the same speed. Cnsider a car mving at 72 km/h and having the crss-sectin area 2 m 2 and drag cefficient 0.3. Cmpute the drag frce acting n the car (a) when T air = 5 C (b) when T air = 40 C Sayfa 14 7

8 Lcal Weather Reprt Turkish State Meterlgical Service (Meterlji Genel Müdürlüğü) Sayfa 15 Heat Heat is energy transferred frm ne bdy t anther by thermal interactins. Heat transfer ccurs whenever there is a temperature difference (ΔT) within an bject between tw bdies between a bdy and its surrundings Heat always flws frm a high-temperature regin t a lw-temperature regin. Heat transfer can ccur in three ways: cnductin, radiatin and cnvectin. Sayfa 16 8

9 SI unit f Energy is Jule (J) 1 J = 1 N.m British Thermal Unit (Btu): 1 Btu = 1055 J Calrie (cal): 1 cal = J SI unit f Pwer (Energy transfer rate) is Watt (W) 1 W = 1 J/s 1 W = Btu/h 1 cal/s = W Sayfa 17 Cnductin Heat cnductin is the transfer f heat energy by micrscpic diffusin and cllisins f particles within a bdy due t a temperature difference. Sayfa 18 9

10 The rate f heat transfer by cnductin is given by Furier s law: where T1 T ka L q 2 q = heat transfer rate (W) k = thermal cnductivity (W/m. C, W/m.K) A = area (m 2 ) L = thickness f the material (m) T 1 T 2 = temperature difference r temperature gradient ( C) Sayfa 19 Sayfa 20 10

11 Example 2 Calculate the heat transfer rate frm a single-pane glass windw as shwn in Fig with an inside surface temperature f 20 C and an utside surface temperature f 5 C. Thermal cnductivity f the The glass is k = 1.4 W/m.K. Sayfa 21 Example 3 Fr a slid fd thermal cnductivity in W/m. C is given by: k = 0.25X c X p X f X a X w where X are the fd mass fractins (X c + X p + X f + X a + X w = 1) (c = "carbhydrate", p = "prtein", f = "fat", a = "ash", and w= "water"). Calculate heat transfer rate fr a fd whse mass fractins are X c =15%, X p =8%, X f =23%, X a =2%, X w =52% and physical parameters are A = 0.2 m 2, ΔT = 25 C and L = 0.05 m. Slutin: k = 0.25X c X p X f X a X w = 0.25(0.15) (0.08) (0.23) (0.02) (0.52) = W/m. C q = k A ΔT / L = (0.391 W/m. C)(0.2 m 2 )(25 C) / (0.05 m) = 39.1 W Sayfa 22 11

12 Thermal Resistance It is imprtant t have a well-insulated huse. Because the better insulated a huse is, the less the heating r cling cst f the huse. T1 T ka L q 2 q T1 T2 L / ka temperatur e difference thermal resistance Here thermal resistance: R L ka Sayfa 23 In engineering practice, the term L/k fr a particular substance is referred t as the R-value f the material. R - value L k Sayfa 24 12

13 Example 4 Determine the thermal resistance and R-value f the glass windw f Example 2. Slutin: R L ka m (1.4 W/m.K)(1.8 m 2 ) K/W R - value L k m (1.4 W/m.K) m 2.K/W Sayfa 25 Multi-Layered Systems Cnsider heat transfer thrugh a cmpsite wall made f several materials f different thermal cnductivities and thicknesses. q Using thermal resistance values fr each layer, we can write: LB k A B B T 1 T LC k A C T1 T R q 2 R C R 2 D LD k A D Sayfa 26 13

14 Example 5 A cld strage wall (3 m x 6 m) is cnstructed f 15 cm thick cncrete (k 2 = 1.37 W/[m. C]). Insulatin must be prvided t maintain a heat transfer rate thrugh the wall at r belw 500 W. The thermal cnductivity f the insulatin is k 1 = 0.04 W/(m. C). The utside surface temperature f the wall is 38 C, and the inside wall temperature is 5 C. (a) Cmpute the required thickness f the insulatin. (Ans: 4.3 cm) (b) Calculate reductin in heat lss. (Ans: 91%) (a) An insulatin with a thickness f 4.3 cm will ensure that heat lss frm the wall will remain belw 500 W. (b) If there is n insulatin, fr q = 500 W, at the end f cncrete, temperature is 35 C. Temperature difference fr cncrete: ΔT = = 3 C New temperature difference: ΔT = 38-5 = 33 C Hence the reductin in heat lss 1-3/33 = Sayfa 27 Heat Capacity The energy Q transferred between a sample f mass m f a material and its surrundings t a temperature change ΔT is given by: Q mc T Where Q = energy transferred (J) m = mass f the sample (kg) c = specific heat capacity (J/kg. C) ΔT = temperature difference ( C r K) Sayfa 28 14

15 Example 6 An aluminum circular disk with a diameter, d = 15 cm and a thickness t = 4 mm is expsed t a heat surce that puts ut 200 J every secnd. The density f the aluminum is 2700 kg/m 3. Assuming n heat lss t the surrunding, estimate the temperature rise f the disk after 20 s. (Ans: ΔT = 24 C) Sayfa 29 Change in Heat Change in heat energy in a fluid stream, if its temperature changes frm T 1 t T 2, is expressed as [3]: Q q Q t mc m c( T 1 T2) t ( T 1 T2) mc ( T 1 T2) where m-dt = mass flw rate f a fluid (kg/s) c = the specific heat f the fluid. The temperature change f a fluid is frm sme inlet temperature T 1 t an exit temperature T 2. Sayfa 30 15

16 Heat Exchanger Cnsider a tubular heat exchanger as shwn in Fig [3]. A ht fluid, H, enters the heat exchanger at lcatin (1) and it flws thrugh the inner pipe, exiting at lcatin (2). A cld fluid, C, enters the annular space between the uter and inner pipes f the tubular heat exchanger at lcatin (1) and exits at lcatin (2). Sayfa 31 Assume that the heat transfer ccurs nly between fluids H and C. Fr H, temperature decreases frm T H,inlet t T H,exit. Fr C, temperature increases frm T C,inlet t T C,exit. The energy balance between fluids: q m H c H ( TH, inlet TH,exit ) m CcC ( TC,exit TC, inlet) Sayfa 32 16

17 Example 7 [see Ref 3] A liquid fd (specific heat 4.0 kj/kg. C) flws in the inner pipe f a duble-pipe heat exchanger. The liquid fd enters the heat exchanger at 20 C and exits at 60 C (see Fig). The flw rate f the liquid fd is 0.5 kg/s. In the annular sectin, ht water at 90 C enters the heat exchanger and flws cunter currently at a flw rate f 1 kg/s. The average specific heat f water is 4.18 kj/(kg. C). Assume steady-state cnditins. (a) Calculate the exit temperature f water. (b) Calculate the energy transferred per unit time t the fluid by water. Sayfa 33 Slutin Given Liquid fd: Inlet temperature = 20 C Exit temperature = 60 C Specific heat = 4.0 kj/(kg. C) Flw rate = 0.5 kg/s Water: Inlet temperature = 90 C Specific heat = 4.18 kj/(kg. C) Flw rate = 1.0 kg/s Sayfa 34 17

18 (a) Using simple heat balance equatin: q m C c C T C m H c H T H kg 0.5 s kj 4 kg C (60 C 20 C) kg 1 s kj 4.18 kg C (90 C TE ) Slving T E = 70.9 C. (b) The rate f energy transfer is: q kg 0.5 s kj 4 kg C (60 C 20 C) 80 kj/s W Sayfa 35 Questins 1. What is the equivalent value f T = 40 C in degrees Fahrenheit, Rankine, and Kelvin? 2. On a summer day, in Gaziantep, the inside rm temperature is maintained at 68 F while the utdr air temperature is a sizzling 110 F. What is the utdr-indr temperature difference in (a) degree Fahrenheit, (b) degree Celsius, and (c) Kelvin? 3. Calculate the heat transfer rate frm a 1000 ft 2, 6-in-thick cncrete wall with inside and utside surface temperatures f 20 C and 0 C. 4. Calculate the amunt f thermal energy required t raise the temperature f 20 galln f water frm 60 F t 120 F. Express yur answer in Btu, J, and cal. 5. Fr Example 4, cnvert the thermal resistance and R-value results frm SI units t British Units. Sayfa 36 18

19 6. We have expsed 1 kg f water, 1 kg f brick, and 1 kg f cncrete, each t a heat surce that puts ut 100 J every secnd. Assuming that all f the supplied energy ges t each material and they were all initially at the same temperature, which ne f these materials will have a greater temperature rise after 10 s? 7. A cpper plate, with dimensins f 3 cm x 3 cm x 5 cm (length, width, and thickness, respectively), is expsed t a thermal energy surce that puts ut 150 J every secnd. The density f cpper is 8900 kg/m3. Assuming n heat lss t the surrunding blck, determine the temperature rise in the plate after 10 secnds. 8. Repeat Example 5 fr q = 1000 W. 9. Repeat Example 7 fr liquid fd with c = 8.0 kj/kg. C. Sayfa 37 References 1. P. Ksky et al., Explring Engineering, 2nd Ed. Elsevier Inc. (2010) 2. S. Maveni, Engineering Fundamentals, 4th Ed. Cengage Learning (2011) 3. R. Paul Singh, Intrductin t Fd Engineering, Fd Science Technlgy, Internatinal Series Sayfa 38 19