Effect Of Longitudinal Heat Conduction Research Articles

The Effect of Longitudinal Heat Conduction on Thermal Enhancement of Plastic Plate Heat Exchangers

The metals are the most popular materials for the plate heat exchangers due to high thermal conductivity. The thermal performance of the metallic plate heat exchanger is degraded by longitudinal heat conduction in the plate. For the cabinet cooling system and ventilator, the metallic plate heat exchanger can be replaced by the plastic plate heat exchanger. In this article, the longitudinal heat conduction in the plastic heat exchanger is studied by using the heat transfer models with and without longitudinal heat conduction, which were proposed in our previous work. The effects of both transverse heat conduction and longitudinal heat conduction can be considered. The thermal performances of the plastic plate heat exchanger and metallic plate heat exchanger are compared. We find out that in the plastic plate, both the transverse heat conduction and longitudinal heat conduction are weakened because of the low thermal conductivity. The high-density polyethylene plastic plate heat exchanger has better thermal performance than that of the aluminum plate heat exchanger.

Thermodynamic characteristics of counter flow and cross flow plate fin heat exchanger based on distributed parameter model

The distributed parameter models of counter flow and cross flow plate fin heat exchanger (PFHE) are constructed to investigate their thermodynamic characteristics. In practical application, the cold end of PFHE may be directly connected to the cold equipment. Therefore, the effect of longitudinal heat conduction (LHC) under different thermal boundary conditions set on two ends of the separating plates (SPs) is investigated based on the model of counter flow PFHE. It is found that LHC effect transmits the effect of boundary condition on two ends of the SPs into the inner domain of PFHE. The LHC effect on the heat transfer performance of PFHE is positive when setting appropriate boundary condition on two ends of the SPs. When considering LHC, compared with the adiabatic boundary conditions on two ends of the SPs, the effectiveness of constant wall temperature boundary conditions increases by 31.7 ∼ 35.8 %, and the effectiveness of constant heat flux boundary conditions increases by 22.8 %. The model of cross flow PFHE is used to investigate the effect of configuration and operation parameters on the heat transfer amount and the pump power. The cross flow PFHE is optimized to arrive at the maximization of heat transfer amount per unit pump power (HTPUP). In the three-stream cross flow PFHE studied, the heat transfer amount almost doesn’t change when the xz projected area is maintained and one projected length (z side length) is changing. The pump power is the lowest and the HTPUP is the largest when z side length of PFHE (z side is the flow direction of the fluid with large mass flowrate) is 0.15 ∼ 0.2 m.

Conjugate Heat Transfer in a Thin Microchannel Filled with a Porous Medium

We study numerically the conjugate heat transfer between the walls of a parallel plate microchannel and a homogeneous porous medium fully saturated with a liquid that is found in motion due to an external pressure gradient. The origin of this problem is caused by a uniform heat flux imposed at the external surfaces of the walls of the microchannel that have a finite thermal conductivity. In this manner, the competition and heat transfer mechanisms between both regions characterized by the thermal resistances, conduct to a conjugate formulation that originates a dimensionless conjugate parameter . This parameter measures the ratio of both thermal resistances, and, for large values of this parameter, the longitudinal heat conduction effects in the walls are very important and suffer significant deviations when compared with the case with finite values for this parameter. The dimensionless governing equations for both regions are established with the corresponding boundary conditions, and the numerical results show that the aspect ratios of both regions, controlled through the dimensionless parameters and , play an important role in distinguishing the presence of the longitudinal heat conduction effects in the walls. For instance, if the ratios and , the longitudinal effects of heat transfer are very important in the walls of the microchannel, whereas in the porous matrix there are effects of heat transfer in both directions, whereas if only the longitudinal conduction effects are significant for both regions.

Thermal Design Considerations and Performance Evaluation of Cryogenic Tube in Tube Heat Exchangers

Heat exchangers are the most important equipment in refrigeration processes. Design and modeling of heat exchangers operating at low temperatures are different from other regular heat exchangers. This study includes two sections. In the first section, design and modeling considerations needed for evaluating the real thermal behavior of heat exchangers at low temperatures were discussed. These considerations are usually neglected by researchers who have modeled the heat exchanger at low temperatures. In the second section, a counter current helically coiled tube in tube heat exchanger operating in hydrogen liquefier was modeled and simulated considering notes discussed in the first section. The model was validated compared with the data presented by literature. The results showed the small positive effect of longitudinal heat conduction on hydrogen liquefaction. The heat in-leak into cold fluid resulted in higher cold fluid outlet temperature and higher hot fluid outlet temperature. Simulations showed that the heat in-leak into cold fluid leads to limit the overdesign for cryogenic heat exchangers. A comparison between models with considering different assumptions was presented and showed that the result may vary significantly based on the regarded assumptions.

A Numerical Study of Small-Scale Longitudinal Heat Conduction in Plate Heat Exchangers

Longitudinal heat conduction has a significant effect on the heat transfer performance of plate heat exchangers, but longitudinal heat conduction is usually neglected in numerical studies and the thermal design of a heat exchanger. In this paper, heat transfer models with and without longitudinal heat conduction are proposed to analyze the effect of small-scale longitudinal heat conduction in a plate heat exchanger. The performance of small-scale longitudinal heat conduction is illustrated by temperature and heat flux contours in the heat transfer models with and without longitudinal heat conduction. The results show that small-scale longitudinal heat conduction occurs in the plate and a more uniform temperature profile of the plate is obtained due to small-scale longitudinal heat conduction. In balanced flow, the contributions of longitudinal heat conduction for counter-flow, cross-flow and parallel-flow plate heat exchangers are −3.15%, −0.09% and 0, respectively, whereas, for the respective unbalanced flows they are evaluated to be −1.73%, 0.53% and 0.05%, respectively. Moreover, it is observed that small-scale longitudinal heat conduction in plates is influenced by the thermal conductivity of the plate. The higher the thermal conductivity, the larger is the reduction of thermal performance. The contribution of longitudinal heat conduction varies from −0.54% to −4.01%.

Numerical analysis of compact plate-fin heat exchangers for aerospace applications

PurposeThe purpose of this study is to find the thermo-hydraulic performances of compact heat exchangers (CHE’s), which are strongly depending upon the prediction of performance of various types of heat transfer surfaces such as offset strip fins, wavy fins, rectangular fins, triangular fins, triangular and rectangular perforated fins in terms of Colburn “j” and Fanning friction “f” factors.Design/methodology/approachNumerical methods play a major role for analysis of compact plate-fin heat exchangers, which are cost-effective and fast. This paper presents the on-going research and work carried out earlier for single-phase steady-state heat transfer and pressure drop analysis on CHE passages and fins. An analysis of a cross-flow plate-fin compact heat exchanger, accounting for the individual effects of two-dimensional longitudinal heat conduction through the exchanger wall, inlet fluid flow maldistribution and inlet temperature non-uniformity are carried out using a Finite Element Method (FEM).FindingsThe performance deterioration of high-efficiency cross-flow plate-fin compact heat exchangers have been reviewed with the combined effects of wall longitudinal heat conduction and inlet fluid flow/temperature non-uniformity using a dedicated FEM analysis. It is found that the performance deterioration is quite significant in some typical applications due to the effects of wall longitudinal heat conduction and inlet fluid flow non-uniformity on cross-flow plate-fin heat exchangers. A Computational Fluid Dynamics (CFD) program FLUENT has been used to predict the design data in terms of “j” and “f” factors for plate-fin heat exchanger fins. The suitable design data are generated using CFD analysis covering the laminar, transition and turbulent flow regimes for various types of fins.Originality/valueThe correlations for the friction factor “f” and Colburn factor “j” have been found to be good. The correlations can be used by the heat exchanger designers and can reduce the number of tests and modification of the prototype to a minimum for similar applications and types of fins.

Theoretical Prediction of Longitudinal Heat Conduction Effects on the Efficiency of the Heat Wheel used for Ventilation in Powerhouse Building “Kjørbo” in Norway

The present study investigates effects of longitudinal heat conduction (LHC) on the heat recovery effectiveness of the heat wheel in the Powerhouse building “Kjørbo”. The effects LHC which are usually ignored in the heat wheel design play a relatively significant role in the heat wheel with high temperature efficiency and short depth in flow direction. The correlation developed by Shah, Kays and London is applied to predict the temperature efficiency and to investigate the impacts of the LHC. Good agreements have been observed between the theoretical predictions obtained from the correlation and experimental data collected from the field test at “Kjørbo”. The influence of the depth of the heat wheel, the airflow rates on the LHC and temperature efficiency were analysed. It was found that the heat wheel has difficulty to achieve a high temperature efficiency (85%) due to the effects of LHC. There is an optimal depth design for heat wheel in the flow direction to recovery maximum thermal energy with low pressure loss with considering the LHC effects. The present study provides preliminary analysis to optimize the heat wheel design and operation with the LHC being taken into account.

The single-blow transient testing technique for offset and wavy fins of compact plate-fin heat exchangers

A single-blow transient test technique is employed to measure the Colburn j factor versus Reynolds number characteristics of high efficiency compact heat exchanger surfaces having offset and wavy fins. Using the experimental data, the NTU values are estimated for 5 types of fins. The measured data is evaluated using the maximum slope method to obtain the corresponding heat transfer coefficient in terms of Colburn j factor, and then is compared with the available correlations of theoretical steady state CFD model. In this analysis, the effects of non-adiabatic side walls and longitudinal heat conduction on the exit fluid temperature response and NTU values are discussed. The test core pressure drops are also measured, and the pressure drop components of entrance, acceleration, core friction and exit losses are compared with the CFD results.

Influence of tube wall longitudinal heat conduction on temperature measurement of cryogenic gas with low mass flow rates

Longitudinal heat conduction is an important parameter in the cryogenic field, especially in cryogenic heat exchangers. In the present study, the parasitic effect of tube wall longitudinal heat conduction on temperature measurement within the tube has been studied for cryogenic gas with low mass flow rates by finite element method and experimental tests. The effects of various parameters such as tube outlet temperature, tube wall thermal conductivity, mass flow rate, and tube wall thickness have been investigated. Axial positioning errors of temperature sensor due to tube wall longitudinal heat conduction were higher for lower gas flow rates. The results showed that the tube wall thermal conductivity leads to axial heat conduction within the tube wall, but the higher tube wall thermal conductivity does not lead to bigger axial positioning error of temperature sensor at tube outlet. According to data obtained from simulations and experiments, sensor with distance of 5mm from tube outlet had 14.92% and 8.51% temperature measurement error (with respect to gas flow temperature at tube outlet) for tube wall thermal conductivities of 16 and 400Wm−1K−1, respectively.

Transient behavior of co-current parallel flow three-fluid heat exchanger

Transient behavior of co-current parallel flow three-fluid compact heat exchangers with the effect of two-dimensional longitudinal heat conduction through the separating sheet and axial dispersion in fluids has been investigated numerically by using the Gauss–Seidel iterative technique for step excitation provided to hot fluid inlet temperature. The results reveal that the performance of the heat exchanger is affected when two-dimensional longitudinal conduction in separating sheets and axial dispersion in fluids are considered.

Effect of longitudinal wall conduction on the performance of a three-fluid cryogenic heat exchanger with three thermal communications

In addition to operating and design parameters, the thermal performance of heat exchangers, working in cryogenic temperature range, is strongly governed by losses such as longitudinal conduction through the wall, heat-in-leak from the surroundings, flow maldistribution, etc. In this paper, a three-fluid cryogenic heat exchanger, involving thermal interaction between all the three-fluids, is investigated for the effect of longitudinal heat conduction in the separating walls on its performance.

Vascularization with line-to-line trees in counterflow heat exchange

Here we report the heat and fluid flow characteristics of counterflow heat exchangers with tree-shaped line-to-line flow channels. The flow structures of the hot and cold sides are sequences of point-to-line trees that alternate with upside-down trees. The paper shows under what conditions the tree vascularization offers greater heat flow access than corresponding conventional designs with parallel single-scale channels. The analytical part is based on assuming fully developed laminar flow in every channel and negligible longitudinal conduction in the solid. The numerical part consists of simulations of three-dimensional convection coupled with conduction in the solid. It is shown that tree vascularization offers greater heat flow access (smaller global thermal resistance) than parallel channels when the number of pairing levels increases and the available pumping power or pressure drop is specified. When the solid thermal conductivity increases, the heat transfer effectiveness decreases because of the effect of longitudinal heat conduction. The nonuniformity in fluid outlet temperature becomes more pronounced when the number of pairing levels increases and the pumping power (or pressure drop number) increases. The nonuniformity in outlet fluid temperature decreases when the solid thermal conductivity increases.

Longitudinal heat conduction study of a uniform power density apparatus for the experimental determination of the heat transfer coefficient

A combined experimental and numerical analysis was performed to accurately evaluate the effect of longitudinal heat conduction along an aluminium test section on the local heat transfer coefficient (HTC) experimental determination. The test section contains a rectangular channel with fins typical of compact heat exchangers. The hydraulic diameter of the channel is around 2 mm. The fluid stream is heated by flat electrical resistances clamped to the outer walls of the test section. The test section contains four zones with different fin pads each, in order to be able to generate data for several geometries at the same time at exactly the same mass flow rate. It was found that longitudinal conduction can lead to a quite uneven distribution of the heat flux to the fluid at the beginning and end of each zone of the test section. Three phenomena were found to be the cause for the observed uneven heat distribution, i.e. the longitudinal gradient of temperature along the wall induced by the fluid temperature rise, the presence of heated and non-heated zones, and the stepwise non-uniformity of the HTC along the test section induced where the fin pad changes. The study has shown that the elementary data reduction methodology, assuming that the surface power density generated by the electrical resistance and the heat flux density reaching the fluid are equal and uniform along the channel, is too far from the reality at the beginning and end of each zone of the test section, especially at low mass flow rates.

Sensitivity analysis and numerical experiments on transient test of compact heat exchanger surfaces

A single-blow transient testing technique considering the effect of longitudinal heat conduction is suggested for determining the average convection heat transfer coefficient of compact heat exchanger surface. By matching the measured outlet fluid temperature variation with similar theoretical curves, the dimensionless longitudinal conduction parameter λ1, the time constant of the inlet fluid temperature τ+, and the number of heat transfer units N tu can be determined simultaneously using the Levenberg-Marquardt nonlinear parameter estimation method. Both sensitivity analysis and numerical experiments with simulated measurements containing random errors show that the method in the present investigation provides satisfactory accuracy of the estimated parameter N tu, which characterizes the heat transfer performance of compact heat exchanger surfaces.

Local effects of longitudinal heat conduction in plate heat exchangers

In a plate heat exchanger, heat transfer from the hot to the cold fluid is a multi-dimensional conjugate problem, in which longitudinal heat conduction (LHC) along the dividing walls often plays some role and can not be neglected. Large-scale, or end-to-end, LHC is always detrimental to the exchanger’s effectiveness. On the contrary, if significant non-uniformities exist in the distribution of either convective heat transfer coefficient, small-scale, or local, LHC may actually enhance the exchanger’s performance by improving the thermal coupling between high heat transfer spots located on the opposite sides of the dividing wall.

The effect of longitudinal heat conduction in cross flow indirect evaporative air coolers

The effect of the longitudinal heat conduction in the exchanger wall of a compact-plate cross flow indirect evaporative cooler is investigated. A NTU method is used to study the heat and mass transfer characteristics. A block iterative numerical method is used to solve the coupled conservation equations for the primary fluid, the secondary fluid and the liquid film. The model was validated using previously published data. The exchanger performance deterioration due to the conduction effect has been determined for various design and operating conditions. The results indicate that the thermal performance deterioration of the evaporative coolers may become significant for some typical operating conditions and could be as high as 10%, while it lies less than 5% for most conservative conditions.

Cooling of a Heat-Generating Strip Immersed in a Laminar Channel Flow

An asymptotic and numerical analysis was conducted for the cooling of a heated strip immersed in a laminar channel flow. The strip material, which can represent an electronic chip, generates heat internally at a uniform rate. Taking into account the finite thermal conductivity of the strip, the nondimensional temperature profile in the strip, the maximum longitudinal temperature differences, and the corresponding Nusselt number have been obtained as functions of the nondimensional parameters a and Pe. The heat conduction parameter a represents the competition between the longitudinal heat conduction in the strip and the heat convection in the laminar cooling flow. The parameter Pe is the well-known Peclet number of the problem. The numerical and asymptotic results for these show a strong dependence of the parameters a and Pe. Therefore, the proposed cooling mechanism serves to identify the role of longitudinal heat conduction effects in the strip. This mechanism can seriously affect the thermal performance of this simplified model of an electronic circuit board.

Forced convection on a heated horizontal flat plate with finite thermal conductivity in a non-Darcian porous medium

The steady-state analysis of conjugated heat transfer process for the hydrodynamically developed forced convection flow on a heated flat plate embedded in a porous medium is studied. The governing equations for the fluid-saturated porous medium are solved analytically using the integral boundary layer approximation. This integral solution is coupled to the energy equation for the flat plate, where the longitudinal heat conduction effects are taken into account. The resulting equations are then reduced to an integro-differential equation which is solved by regular perturbation techniques and numerical methods. The analytical and numerical predictions for the temperature profile of the plate and appropriate local and average Nusselt numbers are plotted for finite values of the conduction parameter, α, which represents the presence of the longitudinal heat conduction effects.

Numerical analysis of the transient conjugated heat transfer in a circular duct with a power-law fluid

We treat numerically in this paper, the tran- sient analysis of a conjugated heat transfer process in the thermal entrance region of a circular tube with a fully developed laminar power-law fluid flow. We apply the quasi-steady approximation for the power-law fluid, identifying the suitable time scales of the process. Thus, the energy equation in the fluids is solved analytically using the well-known integral boundary layer technique. This solution is coupled to the transient energy equation for the solid where the transverse and longitudinal heat conduction effects are taken into account. The numerical results for the temporal evolution of the average tem- perature of the tube wall, hav, is plotted for different nondimensional parameters such as conduction param- eter, a, the aspect ratios of the tube, e and e0 and the index of power-law fluid, n.

Steam condensation waves in water-saturated porous rock

We formulate balance laws governing condensation of steam injected into a linear porous medium containing water. Heat losses to the outside are neglected. Longitudinal heat conduction and capillary effects are taken into account. The condensation process is modeled by a rate equation based on a simple heat-transfer model. We study the condensation front as a traveling wave, under the approximation that pressure variations are negligible within the front. We find this traveling wave for the case of injection of high-quality steam using a combination of phase-plane analysis and numerical calculation of orbits.