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.     

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.     

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.     

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.     

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.     

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.     

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.     

Conjugated heat transfer in double-pass laminar counterflow concentric-tube heat exchangers with sinusoidal wall fluxes

An analytical method is proposed to predict the temperature distribution and local Nusselt number for laminar flow in a double-pass countercurrent heat exchanger with sinusoidal heat flux distribution. A design of inserting in parallel an impermeable barrier to divide an open conduit into two subchannels for conducting double-pass operations, resulting in substantially improved the heat transfer rate, has been evaluated theoretically in the fully developed region. Comparison with the theoretical results shows that the heat-transfer efficiency improvement for double-pass concentric circular heat exchangers is generally higher than those in the single-pass operations without an impermeable barrier inserted. The influences of the impermeable-barrier location on the heat-transfer efficiency improvement and power consumption increment, as indicated from theoretical predictions, can be used to determine the economical feasibility in operating double-pass devices.     

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.     

Recycle effect on heat transfer enhancement in double-pass heat exchangers under asymmetric isotherm conditions

The mathematical formulation of a heat transfer flow problem in the recycling parallel-plate heat exchanger under asymmetric wall temperatures was developed theoretically with ignoring axial conduction, and the analytical solution was obtained using a superposition principle and an orthogonal expansion technique in extended power series. The influences of the design parameters of the impermeable-sheet position (∆), and the operating parameters of the Graetz number (Gz), wall temperature ratio (σ) and recycle ratio (R) are examined. Significant heat transfer improvement is obtainable by employing double-pass devices instead of using single-pass ones for a larger Graetz number system. 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 wall temperature ratios.     

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.     

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 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.     

Enhancement of convective heat transfer in an air-cooled heat exchanger using interdigitated impeller blades

The enhancement of convective heat transfer through a finned heat sink using interdigitated impeller blades is presented. The experimentally investigated heat sink is a subcomponent of an unconventional heat exchanger with an integrated fan, designed to meet the challenges of thermal management in compact electronic systems. The close integration of impeller blades with heat transfer surfaces results in a decreased thermal resistance per unit pumping power. The performance of the parallel plate air-cooled heat sink was experimentally characterized and empirically modeled in terms of nondimensional parameters. Dimensionless heat fluxes as high as 48 were measured, which was shown to be about twice the heat transfer rate of a traditional heat sink design using pressure-driven air flow at the same mass flow rate.     

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.     

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.     

Collector efficiency of upward-type double-pass solar air heaters with fins attached

The collector efficiency of upward-type double-pass flat plate solar air heaters with fins attached and external recycle is investigated theoretically. The double-pass device was constructed by inserting the absorbing plate into the air conduit to divide it into two channels (the upper and lower channels). The double-pass device introduced here was designed for creating a solar collector with heat transfer area double as well as the extended area of fins between the absorbing plate and heated air. Moreover, the advantage of external recycle application to solar air heaters is the enhancement of forced heat convection strength, resulting in considerable device heat transfer performance improvement. This advantage may compensate for the remixing at the inlet which decreases the heat transfer transfer-driving force decrement (temperature difference).     

Nonlinear models for mechanical energy production in imperfect generators driven by thermal or solar energy

In this paper we show how the nonlinear kinetics of energy transfer in propelling fluids and imperfect (non-Carnot) thermal machines can be imbed into the contemporary theory of irreversible energy generators and heat pumps. We quantitatively describe effects of nonlinear heat transfer assuming that heat fluxes are proportional to the difference of temperature in certain power, T^a. We also show that the energy and particle transports can be treated either in a conventional way or as peculiar chemical reactions. In the latter case a recent approach distinguishes in each elementary transfer step two competitive (unidirectional) fluxes and the resulting flux follows as their difference. Nonlinear imperfect systems are investigated in the context of efficiency, heat flux, entropy production and mechanical power, for steady and unsteady operations.     

Performance evaluation of a wavy-fin heat sink for power electronics

The almost daily increase in dissipated power per unit area of electronic components sets higher and higher demands on the performance of the heat sinks. These must not only be able to dissipate high heat fluxes, but must also keep costs to a minimum and exhibit a reliable behaviour. In this paper a novel, modular heat sink consisting of elements with wavy fin profile which can be pressed together to construct the component is presented. Its performance under steady-state conditions are assessed for the case of forced convection in terms of velocity distribution in the channels and global thermal resistance. Configurations with uniform and non-uniform heat flux are studied and some considerations are made as to the influence of the spacers between fan and heat sink proper.