International Journal of Food Engineering

Transcription

1 International Journal of Food Engineering Volume 6, Issue Article 13 Numerical Simulation of Oscillating Heat Pipe Heat Exchanger Benyin Chai, Shandong University Min Shao, Shandong Academy of Sciences Xuanyou Li, Shandong Academy of Sciences Shenjie Zhou, Shandong University Yongchun Shi, Shandong Academy of Sciences Recommended Citation: Chai, Benyin; Shao, Min; Li, Xuanyou; Zhou, Shenjie; and Shi, Yongchun (2010) "Numerical Simulation of Oscillating Heat Pipe Heat Exchanger," International Journal of Food Engineering: Vol. 6: Iss. 1, Article 13. DOI: /

2 Numerical Simulation of Oscillating Heat Pipe Heat Exchanger Benyin Chai, Min Shao, Xuanyou Li, Shenjie Zhou, and Yongchun Shi Abstract Oscillating heat pipe is a new type high efficiency heat-transfer element. Its development and design attract increasing attention. This paper describes a numerical simulation for investigating on the flow and heat exchange performance of an oscillating heat pipe heat exchanger. The influences of the arrangement of heat pipe, the inlet temperature and the flux of air were explored. The results show that the heat exchange of staggered arrangement is more efficient than the aligned one. The influence of temperature difference on the heat exchanger by air flux is more than air inlet temperature. KEYWORDS: field coordinate principle, oscillating heat pipe heat exchanger, numerical simulation

3 Chai et al.: Numerical Simulation of Oscillating Heat Pipe Heat Exchanger Introduction Drying is the oldest food preservation methods. The exhaust gas from a food drying system usually carries a great deal of heat in total. So a proper selection of heat exchangers is very important for heat recovery. As high efficiency heat-transfer element, the heat pipe has been widely used in thermal energy conversion and other fields. It plays an increasingly important role in heat recovery from exhaust in drying system. Oscillating heat pipe, invented by a Japanese scholar H.Akachi (Akachi, 1994; Miyazaki, 1996) in 1994, is a brand new type heat-transfer element. It consists of a long serpentine pipe, straight pipes and elbows. It is ddivided into three sections: heating section, cooling section and insulation section. When the inner diameter of the pipe is small enough, the medium in the vacuum pipe will form plunger. Due to the repeated oscillating caused by interleaving distributing, the high efficiency heat transfer is realized. The oscillating heat pipe not only has good heat transfer capability, but also has higher predominance, simple in structure, flexible in heating mode and heating position compared with traditional heat pipe. Guo and Huang (Chai, 2007) gave the general coordinate principle for an intensified heat exchanger as follows: under the same conditions, a better coordinate between velocity and temperature gradients will cause a stronger heat exchange. The coordinate between velocity field and temperature gradient means: The angle between velocity and temperature gradient should be as small as possible. The velocity and temperature gradients should be parallel; Or velocity, temperature gradient and angle cosine value should be relatively large. For the internal flow in channel, the velocity and temperature distribution on cross-section should be relatively flat. Guo and coworkers (2003, 2004) studied the heat transfer performance of this kind of heat exchanger and drew the following conclusions: the heat transfer performance can be enhanced by improving the temperature distribution of and fluids. The temperature difference between the and fluids does not change with the change of spatial location, that is, temperature difference field is completely uniform. For this situation, it is called a fully coordinated and fluid temperature field, that is, a uniform temperature difference field in terms of their coordinate. The field coordinate principle gives guidelines for the numerical simulation of heat exchange process of oscillating heat pipe heat exchanger. 1

4 International Journal of Food Engineering, Vol. 6 [2010], Iss. 1, Art. 13 Geometrical model Fig.1 (a) and (b) show the build up oscillating heat pipe heat exchanger model of aligned and staggered arrangements. The -air flows through the bottom channel, while the air flows through the top channel. Both cross-sectional areas of the channels are mm 2 with a length of mm. The internal heat transfer mechanism of the oscillating heat pipe is so complicated that, until now, there is no proper method to perform the simulations. Because of the high conductivity of heat pipe and less influence of elbows on heat transfer, the oscillating heat pipes are treated as straight pipes with very large thermal conductivity in this simulation. The thermal conductivity of straight pipe is set to be 10 4 W / (m K). Aligned and staggered arrangements are compared in this paper, as showing in Fig.1 (c) and (d). Figure 1(a, b, c, d) Physical arrangement of the heat pipes: (a)aligned physical model; (b)staggered physical model; (c)aligned pipe arrangements; (d)staggered pipe arrangements DOI: /

5 Chai et al.: Numerical Simulation of Oscillating Heat Pipe Heat Exchanger Figure 2(a, b, c) Meshing: (a) Aligned arrangement; (b) Staggered arrangement; (c) Tube meshing sketch map The non-uniform method was employed to mesh the heat exchanger, as shown in Fig.2 (a) and (b). The mesh near the surface of heat pipes is denser than that of near the wall. Because of the small pipe diameter, the radial conductivity is of less influence on heat exchanging. Therefore there is no radial mesh considered, as shown in Fig.2 (c). Mathematical model The calculations were performed in the three-dimensional rectangular coordinate system. The Reynolds-Averaged Navier-Stokes (RANS) equations were employed to describe the fluid flows. Continuous equation: Momentum equations: x i ( ρu ) = 0 i Dui ρ u u i j 2 u i ρ = + μ + δ + Dt xi x j x j xi 3 x i x j ( ρuu ) ij i j (1) (2) Here, ρ uu i j is Reynolds stress equation tensor, supposed by Boussinesq: u u i j 2 u i ρuu i j = μt + ρk + μt δ ij x j x i 3 xi The turbulence was described by the standard k ε model: (3) 3

6 International Journal of Food Engineering, Vol. 6 [2010], Iss. 1, Art. 13 Dk μ k t ρ = μ + + Gk + Gb ρε YM Dt xi σ k xi 2 Dε μ ε ε ε ( ) Dt x x k k t ρ = μ + C1 ε Gk + C3 εgb C2ε ρ i σ k i Turbulent viscosity coefficient: (4) (5) 2 k = (6) ε μt ρc μ In the above equations, C1 ε = 1.44, C2 ε = 1.92, C μ = The turbulent Prandtl number of turbulent kinetic energy ( k ) and dissipation rate (ε ) is: σ k = 1.0, σ ε = 1.3 Model relates to heat exchange and energy equation is: T ( ρe) + pu ( ρe + p) = ρq + k + τ u t x x x i ij ij i i i Both channels of the heat exchanger were set as with uniform velocity for the air inlet. Except the heat pipes, all the walls were considered as adiabatic boundary. Results and discussion Influence of pipe arrangements (7) In this paper, the influence of heat pipe arrangements on heat exchange was investigated under the following conditions: the air flow rate is 10m 3 /h with an inlet temperature of 370K; the air flow rate is also 10 m 3 /h with an inlet temperature of K. Figs.3 and 4 show the temperature and velocity distributions in the longitudinal sections of and tunnels of aligned and staggered arrangements. It was found that the disturbance is quite weak between a pair of pipes for aligning arrangement. However, the disturbance is significant for staggering arrangement. Obviously, the staggered arrangement has better heat transfer conditions than aligned arrangement. DOI: /

7 Chai et al.: Numerical Simulation of Oscillating Heat Pipe Heat Exchanger Figure 3(a, b) Temperature distribution of -air and -air passage by aligned and staggered arrangement: (a) Aligned; (b) Staggered Figure 4(a, b) Velocity vector chart of -air passage by aligned arrangement: (a) Aligned; (b) Staggered Influence of inlet temperature of air on heat exchange In order to study the effects of air inlet temperature on heat exchange, the simulations were conducted at fixed the inlet temperature (K), air flow rate (10m 3 /h) and air flow rate (10m 3 /h). 5

8 International Journal of Food Engineering, Vol. 6 [2010], Iss. 1, Art. 13 T h =350K Th=350K x [m] x [m] T h =370K T h =370K Th=390K T h =390K x [m] x [m] (a) (b) Figure 5(a, b) Influence of air temperature on heat exchange by aligned and staggered arrangements: (a) Aligned arrangement; (b) Staggered arrangement Fig.5 shows the average temperature profiles along the and tunnels of aligned and staggered arrangements. According to the field coordinate principle, the uniformity of temperature difference field represents their coordinate, that is, more uniform temperature difference field causes better coordinate and better heat exchange effect. It was found that, when the pipes are aligned, the uniformity of temperature difference field does not change obviously with the change of air inlet temperature. It means that the air inlet temperature does not significantly affect the uniformity of temperature difference field. When the -air inlet temperature is 350K, 370K, 390K, the responding average temperature difference is 27.05K, 37.87K, 48.69K and, the DOI: /

9 Chai et al.: Numerical Simulation of Oscillating Heat Pipe Heat Exchanger air outlet temperature is K, K, K in the case of this aligned arrangement. However, the corresponding average temperature difference is 25.39K, 35.55K, 45.71K,and the air outlet temperature is K, K, K in the case of staggered arrangement. It is also observed that both air outlet temperature and air temperature difference increase with the increase in air inlet temperature. It indicates that high air inlet temperature must result in high heat exchange. Comparison with aligned arrangement, under the same inlet temperature, staggered arrangement has a smaller temperature difference between end and end. The above analysis shows that the heat exchange between fluid and pipe of staggered arrangement is better. Influence of air flux In order to trace the effects of air flux on heat exchange only, and air inlet temperature and air flux were fixed at 370K,K and 10m 3 /h, respectively. The air flux studied were: 5 m 3 /h, 10 m 3 /h, 15 m 3 /h and 20 m 3 /h, respectively. The average temperature distribution of and air of aligned and staggered arrangements is shown in Fig. 6. It can be seen that, at the air flux of 10 m 3 /h, the temperature difference between and air is the most uniform for both arrangements. This means, when the air flux is equal to air, the field coordinate is the best. It was also found that the air flux has significant impacts on the coordinate of temperature difference field. It can also be observed from the figure that when the -air flux are 5 m 3 /h, 10 m 3 /h and 20 m 3 /h, the average temperature difference are 34.91K, 37.87K and 39.15K in the case of aligned arrangement and 32.46K, 35.55K, 37.77K of staggered arrangement respectively. This means that the staggered arrangement is of more significant influence on heat exchange. v h =5m3/h v h =5m 3 /h 7

10 International Journal of Food Engineering, Vol. 6 [2010], Iss. 1, Art. 13 v h =10m 3 /h v h =10m 3 /h v h =20m 3 /h v h =20m 3 /h (a) (b) Figure 6(a, b) Influence of air flux on heat exchange by aligned and staggered arrangement: (a) Aligned arrangement; (b) Staggered arrangement Conclusions The flow and heat exchange of oscillating heat pipe heat exchanger has been studied by means numerically. The influences of the arrangement of heat pipes, air inlet temperature and flux on heat exchanger performance were investigated. (1) The arrangement of heat pipe affects the heat exchange significantly, the staggered arrangement is superior to the aligned arrangement. (2) The air inlet temperature is of less effect on the uniformity of temperature difference field, but affects the temperature difference. With the increase in the -air inlet temperature, the temperature difference between and -air increases. (3) The air flux influences the uniformity of temperature difference field greatly. When the air flux is equal to that of the air, the field coordinate is the best. DOI: /

11 Chai et al.: Numerical Simulation of Oscillating Heat Pipe Heat Exchanger References Akachi, H.(1994). Looped Capillary Heat Pipe, Proc. 71st JSME Spring Annual Meeting, Vol.3, No , Chai, B. Y., Li, X. Y., Zhou, SH. J., Liu, D. Y., Guo, X. D. & Li, SH. J.(2007). Experimental Study on Energy Thrift in a Fluidized Bed Dryer With Self-Excited Mode Oscillating-Flow Heat Pipe, The proceedings of the 5 th Asia-Pacific Drying Conference, Hong Kong, Guo, Z. Y. & Huang, S. Y.(2004). Principle of field coordination and new technology of heat transfer enhancement. Beijing, China Electric Power Press. Guo, Z. Y., Wei, SH. & Cheng, X. G.(2003). Principle of field coordination in the enhancement of heat exchanger. Chinese Science Bulletin, 48(22): Miyazaki, Y. & Akachi, H.(1996). Heat Transfer Characteristics of Looped Capillary Heat Pipe, 5th IHPS. 9