N. K. Anand

Use of porous baffles to enhance heat transfer in a rectangular channel

Abstract An experimental investigation was carried out to measure module average heat transfer coefficients in uniformly heated rectangular channel with wall mounted porous baffles. Baffles were mounted alternatively on top and bottom of the walls. Heat transfer coefficients and pressure loss for periodically fully developed flow and heat transfer were obtained for different types of porous medium (10, 20, and 40 pores per inch (PPI)) with two window cut ratios (Bh/Dh=1/3 and 2/3) and two baffle thickness to channel hydraulic diameter ratios (Bt/Dh=1/3 and 1/12). Reynolds number (Re) was varied from 20,000 to 50,000. To compare the effect of foam metal baffle, the data for conventional solid-type baffle were obtained for (Bt/Dh=1/3). The maximum uncertainties associated with module Nusselt number and friction factor were 5.8% and 4.3% respectively. The experimental procedure was validated by comparing the data for the straight channel with no baffles (Bh/Dh=0) with those in the literature [Publications in Engineering, vol. 2, University of California, Berkeley, 1930, p. 443; Int. Chem. Eng. 16 (1976) 359]. The use of porous baffles resulted in heat transfer enhancement as high as 300% compared to heat transfer in straight channel with no baffles. However, the heat transfer enhancement per unit increase in pumping power was less than one for the range of parameters studied in this work. Correlation equations were developed for heat transfer enhancement ratio and heat transfer enhancement per unit increase in pumping power in terms of Reynolds number.

The effect of plate spacing on free convection between heated parallel plates

In the past, considerable attention has been given to free convection between heated vertical parallel plates. This problem is considerable interest to engineers because of its application to electronic equipment cooling and solar energy. Some attempts have been made to optimize the spacing between parallel plates in the past. Bodoia and Osterle analytically derived a criterion for an optimum plate spacing for which the heat dissipation is maximum. The objective of this paper is to predict the optimum plate spacing for single channels by using the governing equations for a continuous system model. No heat transfer coefficient known a priori will be used in these calculations, but will be calculated as part of the solution.

Laminar developing flow and heat transfer between a series of parallel plates with surface mounted discrete heat sources

Abstract Laminar developing flow (DF) and heat transfer between a series of conducting parallel plates (substrate) with surface mounted heat generating blocks were numerically studied with consideration given to flow of air (Pr = 0.7). These channels resemble cooling passages of electronic equipment. A single channel subjected to a repeated condition in the transverse direction was isolated as a computational domain. The governing equations were solved by a finite volume technique. The results of the DF problem were compared with the corresponding periodically fully developed flow (PDF) problem results and used to establish entry length. Thermal entry length decreased with an increase in substrate conductivity. Thermal performance of channels was characterized in terms of thermal resistance per unit length of the channel. To separate the effects of self heating and upstream heating for each module, thermal resistance based on the channel inlet temperature ( R o ) and module inlet bulk temperature ( R m ) was defined. These thermal resistances were correlated with the independent parameters such as the Reynolds number (Re), substrate thickness ( t w ), block height ( h w ), block spacing ( s w ), channel height ( d w ), and thermal conductivity ratio of the solid to fluid ( K s K f ). The thermal resistance was found to decrease with an increase in Reynolds number, block spacing and substrate conductivity. The thermal resistance increased with an increase in the area of bypass flow ( 1− h d ), substrate thickness ( t w ) and block height ( h w ).

Effect of wall conduction on free convection between asymmetrically heated vertical plates: uniform wall heat flux

Abstract In this study of the effect of wall conduction on laminar free convection between asymmetrically heated vertical plates, an implicit finite difference scheme is used to solve the governing equations. The governing independent parameters are identified to be Prandtl number ( Pr ), Grashof number ( Or ), ratio of thermal conductivity of the solid to air (K), wall thickness to channel width ratio ( t / B ), channel height to width ratio ( L / B ) and the asymmetric heating parameter ( γ H ). The effect of wall conduction on free convective flow of air under asymmetrically heated conditions ( γ H = 1.0, 0.5, and 0) is discussed. Calculations are made for K = 1 and 10, t / B = 0.1 and 0.5, Gr = 10-10 4 , and L / B = 1 and 5. The maximum increase in mass flow rate of air for symmetric heating due to wall conduction is 30%. The maximum decrease in average Nusselt number due to wall conduction is 22%. Wall conduction effects are more significant for low Gr flows than for high Gr flows.

HEAT TRANSFER IN A THREE-DIMENSIONAL CHANNEL WITH BAFFLES

A numerical investigation of laminar forced convective heat transfer was performed in a three-dimensional channel with baffles in which a uniform heat flux was applied to the top and bottom walls, and the sidewalls were considered adiabatic. The trade-off between heat transfer enhancement and pressure drop produced by the baffles was studied for periodically fully developed flow (PDF). The numerical analysis was performed using a finite volume approach. The computer code was validated against the experimental results of Goldstein and Kreid [1] and Beavers et al. [2] for a three-dimensional channel without baffles. Parametric runs were made for Reynolds numbers of 150, 250, 350, and 450 for baffle height to channel width ratios (H/D y ) of 0.5, 0.6, 0.7, and 0.8. Heat transfer behavior was studied for Prandtl numbers of 0.7 and 7.0, and for wall thermal conductivity to fluid thermal conductivity ratios (K) of 1, 10, 100, and 1000.

NUMERICAL STUDY OF HEAT AND MOMENTUM TRANSFER IN CHANNELS WITH WAVY WALLS

ABSTRACT A two-dimensional steady developing fluid flow and heat transfer through a periodic wavy passage were studied numerically for a fluid with a Prandlt number of 0.7 and compared to flow through a corresponding straight (parallel-plate) channel. Sinusoidal and arc-shaped configurations were studied for a range of geometric parameters. At low Reynolds number, the two geometric configurations showed little or no heat transfer augmentation in comparison with a parallel-plate channel. In some cases the heat transfer enhancement ratios were as high as 80% at higher Reynolds number. An increase in either the height ratio or the length ratio for both sine and arc-shaped configurations resulted in a decrease in the recirculation size and strength. Periodically fully developed flow was attained downstream of the first module of the six modules considered in this study.

CONVECTIVE HEAT TRANSFER IN A CHANNEL WITH POROUS BAFFLES

Laminar forced-convective heat transfer in a two-dimensional parallel-plate channel with 16 porous baffles mounted alternately on bottom and top walls was studied numerically. The numerical study was conducted by developing and using a finite-volume code. The pressure and velocity fields were linked by the SIMPLEC algorithm. The extended Darcy-Forchheimer model was used to describe resistance to flow through the porous baffles. The grid independence was established for the developed code. The code was validated against the studies by Sung et al. [7] and Nakayama [15]. The parametric runs were made for Reynolds numbers (Re) of 100, 200, 300, and 400; for Darcy number (Da) values of 8.783 × 10−6, 1.309 × 10−5, and 1.791 × 10−5; for nondimensional baffle spacing values of (D*)=11, 13, and 15; for nondimensional baffle aspect ratio (W*) of 4, 6, and 12; thermal conductivity ratios (K*) of 1, 10, and 100; and nondimensional baffle height (B*) was fixed at 1/3. Consideration was given only to flow of air (Pr=0.7). It was found that heat transfer enhancement ratios for solid-baffle cases are higher than those for corresponding porous-baffle cases. The heat transfer enhancement ratio increased with increase in Re, decrease in Da, increase in K*, increase in D*, and decrease in W*. The heat transfer enhancement per unit increase in pumping power was less than unity for the range of parameters considered in this study.

Turbulent Heat Transfer Between a Series of Parallel Plates With Surface-Mounted Discrete Heat Sources

Two-dimensional turbulent heat transfer between a series of parallel plates with surface mounted discrete block heat sources was studied numerically. The computational domain was subjected to periodic conditions in the streamwise direction and repeated conditions in the cross-stream direction (Double Cyclic). The second source term was included in the energy equation to facilitate the correct prediction of a periodically fully developed temperature field. These channels resemble cooling passages in electronic equipment. The k–e model was used for turbulent closure and calculations were made for a wide range of independent parameters (Re, Ks /Kf , s/w, d/w, and h/w). The governing equations were solved by using a finite volume technique. The numerical procedure and implementation of the k–e model was validated by comparing numerical predictions with published experimental data (Wirtz and Chen, 1991; Sparrow et al., 1982) for a single channel with several surface mounted blocks. Computations were performed for a wide range of Reynolds numbers (5 × 104 –4 × 105 ) and geometric parameters and for Pr = 0.7. Substrate conduction was found to reduce the block temperature by redistributing the heat flux and to reduce the overall thermal resistance of the module. It was also found that the increase in the Reynolds number decreased the thermal resistance. The study showed that the substrate conduction can be an important parameter in the design and analysis of cooling channels of electronic equipment. Finally, correlations for the friction factor (f) and average thermal resistance (R) in terms of independent parameters were developed.

Free Convection Between Series of Vertical Parallel Plates With Embedded Line Heat Sources

Laminar free convective heat transfer in channels formed between series of vertical parallel plates with an embedded line heat source was studied numerically. These channels resemble cooling passages in electronic equipment. The effect of a repeated boundary condition and wall conduction on mass flow rate (M), maximum surface temperature ({theta}{sub h,max} and {theta}{sub c,max}), an average surface Nusselt number (Nu{sub h} and Nu{sub c}) is discussed. Calculations were made for Gr* = 10 to 10{sub 6}, K = 0.1, 1, 10, and 100, and t/B = 0.1 and 0.3. The effect of a repeated boundary condition decreases the maximum hot surface temperature and increases the maximum cold surface temperature. The effect of a repeated boundary condition with wall conduction increases the mass flow rate. The maximum increase in mass flow rate due to wall conduction is found to be 155%. The maximum decrease in average hot surface Nusselt number due to wall conduction (t/B and K) occurs at Gr* = 10{sup 6} and is 18%. Channels subjected to a repeated boundary condition approach that of a symmetrically heated channel subjected to uniform wall temperature conditions at K {ge} 100.

Effect of short-tube orifice size on the performance of an air source heat pump during the reverse-cycle defrost

Abstract Many air source heat pumps use the reverse-cycle defrost to eliminate frost that forms on the outdoor heat exchanger during normal winter operation. During the defrost, the heat pump is switched from the heating to the cooling mode to provide heat to the outdoor heat exchanger to melt the frost. Once the frost is melted and drained from the heat exchanger, the unit is switched back to the heating mode. The objective of this research was to characterize the effect of short-tube orifice diameter on the response of a heat pump during the reverse-cycle defrost. An experimental apparatus was constructed containing a nominal 3 ton (cooling capacity) residential air source heat pump. A manifold was constructed which allowed for the switching of different sized orifices by turning a shut-off valve. Refrigerant temperature and pressure measurements were made throughout the system as well as refrigerant flow-rates, air-side capacity, compressor/outdoor fan power and refrigerant level in the accumulator. A detailed comparison of the effect on defrost performance is provided.