Double-pass flow heat transfer in a parallel-plate channel for improved device performance under uniform heat fluxes

A design of inserting in parallel an impermeable sheet to divide an open conduit into two subchannels for conducting double-pass laminar countercurrent operations under uniform heat fluxes, resulting in substantially improved the heat-transfer rate, has been designed and investigated theoretically by using an eigenfunction expansion in terms of power series for the homogeneous part and an asymptotic solution for the non-homogeneous part. The theoretical results of heat-transfer efficiency enhancement in double-pass parallel-plate heat exchangers are represented graphically and compared with those in the single-pass operations without an impermeable sheet inserted. The influence of the impermeable-sheet location on the heat-transfer efficiency enhancement as well as on the power consumption increment in double-pass operations has also been delineated.     

Asymmetric wall heat fluxes boundary conditions in double-pass parallel-plate heat exchangers

Boundary conditions for asymmetric wall heat fluxes in double-pass parallel-plate laminar countercurrent operations are analyzed theoretically in this work by using an eigenfunction expansion in terms of power series for the homogeneous part and an asymptotic solution for the non-homogeneous part. Effects of variable ratio of heat fluxes on both sides and impermeable-sheet location are also studied. Quantitative and qualitative interpretations of theoretical predictions are utilized to investigate of heat-transfer efficiency enhancement of the double-pass model under consideration as compared to those in the single-pass operations without an impermeable sheet inserted. Results are presented in terms of Nusselt number and device performance improvement. The influence of the impermeable-sheet location on the heat-transfer efficiency enhancement as well as on the power consumption increment in double-pass operations has also been delineated.     

Recycle effect on heat transfer enhancement in double-pass parallel-plate heat exchangers under asymmetric wall fluxes

A new design of conjugated heat transfer in double-pass parallel-plate laminar countercurrent operations subjected to asymmetric wall heat fluxes has been investigated theoretically, and the analytical results obtained with superposition model which introduces the an eigenfunction expansion in terms of power series for the homogeneous part and an asymptotic solution for the inhomogeneous part. The influences of the design parameters, the impermeable-sheet position (Δ), and the operating parameters, the mass-transfer Graetz number (Gz), wall heat flux ratio (Q_r) and recycle ratio (R), are examined. The results are represented graphically and compared with those in a single-pass parallel-plate heat exchanger (without inserting an impermeable sheet). Considerable improvement in heat transfer is obtainable by employing such a recyclic double-pass device, instead of using the single-pass one. A technical feasibility of the new double-pass device was investigated in terms of the Nusselt number and device performance improvement under the effect of variable ratio of heat fluxes on both sides. It concluded that a substantial heat-transfer efficiency improvement is achieved by employing such a recyclic device with suitable elections of impermeable-sheet position and recycle ratio.     

The influences of recycle on a double-pass laminar counterflow concentric circular heat exchangers

A new device of inserting an impermeable sheet with negligible thermal resistance to divide a circular tube into two subchannels with uniform wall temperature and external refluxes at the ends, resulting in substantially improving the heat transfer, has been designed. The mathematical formulation and theoretical analysis to such a conjugated Graetz problem of double-pass concentric circular heat exchangers have been developed by the use of an orthogonal expansion technique. The analytical results are represented graphically and compared with that in an open conduit of the same size without recycle. Considerable improvement in heat transfer is obtainable by employing double-pass operations with inserting an impermeable sheet instead of using single-pass operations without external refluxes. Two numerical examples in heat transfer efficiency by arranging the recycle effect as well as the power consumption were illustrated. The effects of the channel thickness ratio on the enhancement of heat transfer efficiency as well as on the power consumption increment have been also discussed.     

The influences of recycle on a double-pass laminar counterflow concentric-tube heat exchangers with sinusoidal wall fluxes

The effects of external recycle at the ends on double-pass laminar countercurrent heat exchangers with sinusoidal heat flux distribution are investigated analytically by setting a general solution to separate the original boundary value problem into a partial differential equation, which is solved by Frobenius method, and an ordinary differential equation. Analytical results show that recycle effects enhance the heat-transfer efficiency due to that the desirable effect of forced-convection increment has more influence than the undesirable effect of the heat-transfer driving-force decrement, and hence the forced-convection increment by increasing the recycle ratio leads to improved device performance in heat-transfer rate as compared with that in the single-pass operation (without an impermeable sheet inserted).     

An analytical study of power-law fluids in double-pass heat exchangers with external recycle

The influence of the recycle effect on heat transfer to the power-law fluids from a double-pass parallel-plate heat exchanger with uniform wall temperature has been studied analytically. The governing equations are solved by an orthogonal expansion technique in terms of power series. The heat transfer problem is solved for fully developed laminar flow through parallel-plate channels by ignoring axial conduction, with fluid properties of temperature independence. The dependence of the average Nusselt number on the Graetz number has been studied in detail. Analytical results show that the recycle effect can enhance the heat transfer efficiency as compared with that in a single pass (without an impermeable plate inserted and without recycle). Considerable heat-transfer enhancement is obtained by introducing the recycle-effect concept in designing such double-pass operations. The effects of the impermeable-sheet position and the power-law index on the enhancement of device performance as well as the power consumption increment have also been delineated.     

Improvement in performance on laminar counterflow concentric circular heat exchangers with external refluxes

Considerable improvement of heat transfer in laminar counterflow concentric heat exchangers is obtainable by inserting in parallel an impermeable, resistless sheet to divide an open duct into two subchannels for double-pass operations with external refluxes. Efficiency improvement in heat transfer has been investigated analytically by using an orthogonal expansion technique. The results of improvement in heat transfer efficiency are represented graphically and compared with those in a single-pass operation. The influences of improvement-sheet location and reflux ratio on the enhancement of transfer efficiency as well as on the increment of power consumption have been discussed.     

Improvement in device performance on laminar counterflow concentric circular heat exchangers with uniform wall fluxes

The effects of recycle at the ends on the heat transfer through a concentric circular tube with uniform wall fluxes are studied analytically by an orthogonal expansion technique with eigenfunction power series expansion. The heat transfer problem is solved for fully developed laminar velocity profiles in a double-pass circular heat exchanger with ignoring axial conduction and fluid properties of temperature independence. Analytical results show the external recycle can enhance the heat transfer efficiency compared with that in an open tube (without an impermeable circular tube inserted and without recycle). The compensation between the forced-convection increment and heat-transfer driving force decrement are used to study the heat transfer behavior. The effects of the impermeable-tube location on heat transfer efficiency enhancement as well as the power consumption increment have been also discussed.     

Heat transfer of conjugated Graetz problems with laminar counterflow in double-pass concentric circular heat exchangers

A device of external recycle at the ends of double-pass concentric circular heat exchangers with uniform wall temperature, resulting in substantially improving the heat transfer, has been designed and studied theoretically. The theoretical analysis on heat transfer efficiency improvement has been developed using orthogonal expansion technique in power series. The analytical results are also represented graphically and compared with that in an open conduit (without an impermeable plate inserted and without recycle). Considerable improvement in heat transfer is obtainable by employing the external recycle at both ends with a suitable adjustment of the impermeable-sheet position and recycle ratio, instead of using an open conduit.     

Recycle effect on heat-transfer efficiency improvement in a double-pass parallel-plate heat exchanger under uniform wall fluxes

The recycle concept was applied to a double-pass parallel-plate heat exchanger under uniform wall fluxes to enhance the device performance improvement. The mathematical statement of the conjugated Graetz problem was developed theoretically and the analytical solution was obtained by using an eigenfunction expansion in terms of power series for the homogeneous part and an asymptotic solution for the inhomogeneous part. The theoretical predictions of wall temperature and average Nusselt numbers are represented graphically with Graetz number and impermeable-sheet location as parameters. A substantial heat-transfer efficiency improvement is achieved by employing such a recyclic device with suitable adjustments of the impermeable-sheet position and recycle ratio, instead of using the single-pass device. The results indicate that the recyclic operation can effectively enhance the heat transfer efficiency, especially when the recyclic device was operated in large Graetz number.     

The influences of recycle on performance of baffled double-pass flat-plate solar air heaters with internal fins attached

A new device for inserting an absorber plate to divide a flat-plate channel into two parts with fins attached by baffles and external recycling at the ends is presented. The proposed device substantially improves the heat-transfer efficiency. Experimental and theoretical investigations into the device efficiency are presented. The theoretical prediction agreement with the measured values from the experimental results is good. The experimental and theoretical results are represented graphically and compared with data from the downward-type single-pass solar air heaters of the same size without recycling. Considerable heat-transfer improvement is obtained by employing baffled double-pass operations with external recycling and fin attached over and under the absorber plate. The recycle ratio and absorber plate location influences on the heat-transfer efficiency and on the power consumption increment are also discussed.     

The influence of recycle on a parallel-plate heat exchanger with insulation sheet inserted for double-pass operations

The influence of recycle on a parallel-plate heat exchanger of inserting in parallel an insulation sheet to divide an open duct into two channels for double-pass operations with uniform wall temperature has been studied analytically. The results are represented graphically and compared with those in an open duct (without an inserted insulation sheet) and a double-pass without recycle. Substantial improvement in heat transfer is obtainable by employing such a double-pass device with external refluxes, instead of using an open conduit with single-pass operations and using double-pass operations without recycle. The effect of insulation-sheet location on the enhancement of heat transfer efficiency as well as on the increment of power consumption, has been also discussed.     

Heat-transfer efficiency improvement of double-pass concentric circular heat exchangers under uniform wall fluxes

The new design of the double-pass operation with external recycle is proposed to apply to enhance the heat-transfer efficiency of concentric circular heat exchangers under uniform wall fluxes. The mathematical formulation was developed theoretically and the analytical solutions were achieved by using the orthogonal expansion technique with the eigenfunctions expanding in terms of an extended power series. The theoretical predictions were represented graphically and compared with those obtained from the single-pass device without external recycle and the previous work [C.D. Ho, S.C. Yeh, Improvement in device performance on laminar counterflow concentric circular heat exchangers with uniform wall flux, International Journal of Heat and Mass Transfer 49 (2006) 202–2032] under the different flow pattern. The results show that a considerable heat-transfer efficiency improvement was obtained with a suitable adjustment of the subchannel thickness ratio and recycle ratio. The power consumption increment due to the device with external recycle was also discussed in this study.     

Heat-transfer enhancement in double-pass flat-plate solar air heaters with recycle

This work theoretically and experimentally investigates a device for inserting an absorbing plate into the double-pass channel in a flat-plate solar air heater with recycle. This method substantially improves the collector efficiency by increasing the fluid velocity. The results are represented graphically and compared with a downward-type single-pass solar air heater. Considerable improvement in heat transfer is obtainable by employing recycle-type double-pass devices instead of single-pass devices or a conventional double-pass heater with the same flow rate. The absorbing plate location influence on heat-transfer efficiency enhancement and the hydraulic dissipated power increment is also discussed.     

Heat transfer enhancement for the power-law fluids through a parallel-plate double-pass heat exchangers with external recycle

The conjugated Graetz problem of the double-pass heat transfer through a parallel-plate device with uniform wall temperature and external recycle in the outlet channel was solved analytically with the use of the orthogonal expansion technique for the power-law fluids. The mathematical formulation of the heat transfer problem was developed for fully developed laminar velocity profiles through the parallel-plate channels by ignoring axial conduction and assuming fluid properties of temperature independence. The constant outer wall temperature and continuous temperature and heat flux between the two subchannels with inserting impermeable sheet were considered for thermal boundary conditions. The analytical results show that the recycle ratio and impermeable sheet position play significant influences on the efficiencies of the heat transfer as compared to that in a single pass (without an impermeable sheet inserted and without recycle). The outlet temperature of the heat exchanger seems to be independent of the power-law index of the fluid, while the average Nusselt number could not be enhanced significantly with the lowering power index. The power consumption increment owing to the cross-sectional area reduction from single-pass to double-pass was also taken into account for comparisons.     

Optimization principles for convective heat transfer

Optimization for convective heat transfer plays a significant role in energy saving and high-efficiency utilizing. We compared two optimization principles for convective heat transfer, the minimum entropy generation principle and the entransy dissipation extremum principle, and analyzed their physical implications and applicability. We derived the optimization equation for each optimization principle. The theoretical analysis indicates that both principles can be used to optimize convective heat-transfer process, subject to different objectives of optimization. The minimum entropy generation principle, originally derived from the heat engine cycle process, optimizes the convective heat-transfer process with minimum usable energy dissipation focusing on the heat–work conversion. The entransy dissipation extremum principle however, originally for pure heat conduction process, optimizes the heat-transfer process with minimum heat-transfer ability dissipation, and therefore is more suitable for optimization of the processes not involving heat–work conversion. To validate the theoretical results, we simulated the convective heat-transfer process in a two-dimensional foursquare cavity with a uniform heat source and different temperature boundaries. Under the same constraints, the results indicate that the minimum entropy production principle leads to the highest heat–work conversion while the entransy dissipation extremum principle yields the maximum convective heat-transfer efficiency.     

Thermal and Thermohydraulic Efficiency of Recyclic-Type Double-Pass Solar Air Heaters With Fins and Baffles

This paper presents investigations related to the heat-transfer performance improvement of double-pass finned and finned with baffles solar air heaters. The analytical study has been carried out to see the effects of parameters such as mass flow rate, recycle ratio, fins thickness, number of fins, and baffles width on the heaters’ efficiencies. It is indicated that the mass flow rate and the recycle ratio are the most significant factors, which considerably increase the heaters’ efficiencies by increasing the fluid velocity. The theoretical results showed that the device with fins plus baffles can enhance the heat transfer rate compared with the device with fins under the same flow rate and working dimensions. The effect of the depth of the first channel on the thermal and thermohydraulic efficiency improvement is also delineated. To validate the proposed theoretical models, comparisons of the results are performed with those obtained from previous studies and show that good agreement is achieved.     

Numerical simulation of heat transfer in regenerator of solid adsorption refrigeration system

In this paper a theoretical model of the heat-transfer processes in a solid adsorbent packed bed is established. Based on discretized energy control equations of the fluid in the heat-transfer coil and transient heat conduction equations of the adsorbent in the bed with unsteady boundary conditions, a numerical analysis is made. Through numerical computation, a coupling temperature distribution in the adsorbent and the heat-transfer coil is obtained. This will make for an optimal design of the solid adsorbent packed bed.     

Effect of heat recovery on the performance of a shape-memory effect heat engine

The performance of a shape-memory effect heat engine employing heat recovery has been modelled taking into account heat transfer to the working element and non-ideal material behaviour. Calculations of the energy flux to and from the working element show that a substantial fraction of the sensible heating requirements can be recovered by heat transfer between the heating and cooling stages. The increase in efficiency due to heat recovery and the resulting power output are shown to be a function of stress and rate of temperature change of the heat-transfer fluid. Working element-fluid heat transfer and hysteresis behaviour of the shape-memory working element are found to limit the efficiency improvement associated with heat recovery.