Heat transfer enhancement in permeable tubes subjected to transverse suction flow is investigated in this work. Both momentum and energy equations are solved analytically and numerically. Both solutions based on negligible entry regions are well matched. Two different suction velocity distributions are considered. A parametric study including the influence of the average suction velocity and the suction velocity profile is conducted for various Peclet numbers. It is found that enhancement of heat transfer over that in impermeable tubes is only possible with large Peclet numbers. This enhancement increases as suction velocities towards the tube outlet increase and as those towards the tube inlet decrease simultaneously. The identified enhancement mechanisms are expanding the entry regions, increasing the transverse advection, and increasing the downstream excess temperatures under same transverse advection. The average suction velocity that produces maximum enhancement increases as the Peclet number increases until it reaches asymptotically its uppermost value at large Peclet numbers. The maximum reported enhancement ratios for the exponential and linear suction velocity distributions are 17.62-fold and 14.67-fold above those for impermeable tubes, respectively. This work demonstrates that significant heat transfer enhancement is attainable when the suction flow inside the permeable tubes is distributed properly.

Laminar flow and heat transfer in different protruding-edged plate systems are modelled and analyzed in the present work. These include the Parallel Flow (PF) and the Counter Flow (CF) protruding-edgedplate exchangers as well as those systems being subjected to Constant Wall Temperature (CWT) and Uniform Heat Flux (UHF) conditions. These systems are subjected to normal free stream having both power-law velocity profile and same average velocity. The continuity, momentum and energy equations are transformed to either similarity or nonsimilar equations and then solved by using well validated finite difference methods. Accurate correlations for various flow and heat transfer parameters are obtained. It is found that there are specific power-law indices that maximize the heat transfer in both PF and CF systems. The maximum reported enhancement ratios are 1.075 and 1.109 for the PF and CF systems, respectively, at Pr = 100. These ratios are 1.076 and 1.023 for CWT and UHF conditions, respectively, at Pr = 128. Per same friction force, the CF system is preferable over the PF system only when the power-law indices are smaller than zero. Finally, this work demonstrates that by appropriately distributing the free stream velocity, the heat transfer from a plate can be increased up to 10% fold.

Laminar falling film condensation over vertical short fin is analyzed. The fin is considered to be hollow and permeable in order to account for condensate suction towards its inner core. The present work accounts for the shear stress at the interface caused by the large velocity of the saturated vapor. The continuity, momentum, and energy equations for the film condensate are solved using an iterative and implicit finite-difference method. The condensate flow rate inside the fin is obtained using the conservation of mass principle and the Poiseuille flow equations. The one-dimensional fin equation model is used to relate the fin temperature to the condensate flow convection heat transfer coefficient. Various computational and numerical methods techniques such as advanced linearization and generations of best-fit correlations are implemented. This is to reduce significantly the number of iterations required for solution convergence as all of the aforementioned equations are coupled. It is found that the dimensionless total mass transfer rate increases slightly as vapor Reynolds number increases. It increases apparently as both suction Reynolds number and dimensionless suction length increase. For both large suction Reynolds number and large dimensionless suction length, the flow rate of the condensate in the falling film can be neglected compared to the total suction condensate flow rate. The fin thermal length increases as Reynolds number, dimensionless suction length, and Grashof number increase. This study shows that significant condensation mass flow rate enhancement ratios are obtainable (can be 18 folds) due to suction through a hollow super conductive fin.

This work analyzes heat transfer through a wall containing triangular fins partially embedded in its volume. The coupled heat diffusion equations governing each constituent are solved numericallyusing an iterative finite volume method. A well bracketed effectiveness of the combined system that suits wide range of applications is analytically derived. Good agreement between the numerical and the analytical results is attained. It is found that the fin-root can act simultaneously as a heat sink and heat source for the wall. The heat transfer rate through the combined system is clearly seen to be maximized at a specific fin-root length. The maximum reported heat transfer rate through the triangular rooted-finned wall is found to be at most 90% above that for the rootless fin case at wall Biot number of 1.54. This percentage is noticed to decrease as the wall Biot number decreases. At that Biot number, the maximum heat transfer rate through the combined system reaches 150% above that through the plain wall. As a result of this work, it is recommended to utilize the triangular rooted-fin as a heat transfer enhancer for high mechanical strength structures exposed to highly convective fluid streams.

Heat transfer enhancement in falling two immiscible co-flows arranged concentrically inside a vertical tube is investigated in this work. The momentum and energy equations of both fluids are solved analytically and numerically. The numerical and the analytical results based on adiabatic and zero-shear-stress interfacial conditions are well matched. A parametric study including the influence of fluids relative densities, viscosities, thermal conductivities, specific heats and the flows relative radii is conducted for various reference Reynolds numbers. Different ranges of the fluids thermophysical properties that augment heat transfer are obtained and discussed. When the (inner, outer) fluids pair is given as (water, mercury), the maximum enhancement factor is found to be 1.029 fold relative to the case when the outer fluid filling the whole volume. Interchanging the fluids of that pair can increase the enhancement factor up to 4.58 fold due to flow rate amplification caused by the decrease in the friction force. The use of (air, mercury) pair can increase the enhancement ratio to above 2.82 fold if the flows are thermally fully developed due to significant reduction in effective viscosity. This work demonstrates that significant heat transfer enhancement is attainable when combining layering of immiscible fluids and flow rate amplification mechanisms.

Enhancement of heat transfers in counterflow plate heat exchanger due to presence of an intermediate auxiliary fluid flow is investigated. The intermediate auxiliary channel is supported by transverse conducting pins. The momentum and energy equations for the primary fluids are solved numerically and validated against a derived approximate analytical solution. A parametric study including the effect of the various plate heat exchanger, and auxiliary channel dimensionless parameters is conducted. Different enhancement performance indicators are computed. The various trends of parameters that can better enhance heat transfer rates above those for the conventional plate heat exchanger are identified. Large enhancement factors are obtained under fully developed flow conditions. The maximum enhancement factors can be increased by above 8.0- and 5.0-fold for the step and exponential distributions of the pins, respectively. Finally, counterflow plate heat exchangers with auxiliary fluid flows are recommended over the typical ones if these flows can be provided with the least cost.

Enhancement of heat and exergy transfers in double pipe exchangers with conical tubes is investigated. Different fluid combinations are selected. The momentum and energy equations for both fluids are solved mathematically and numerically. Both solutions are well matched. A parametric study including the effect of the thermal conductivity, thermal capacity and the tube to pipe diameters ratios is conducted. Different performance indicators are computed including the second-law efficiency. Large enhancement factors are obtained when the fluid of smaller thermal conductivity is placed in the annulus. For this case, heat and exergy transfers' enhancement factors may increase to 2.0 and 2.38 folds, respectively. Under same input power, the convergent tube increases heat and exergy transfers above those for conventional exchanger when thermal entry-regions are insignificant. However, the divergent tube does that when entry-regions are substantial. Finally, using conical tubes to enhance heat and exergy transfers inside double pipe exchangers are recommended.

Analysis of heat transfer through different candidates for high performance fins is considered in this work. These candidates are: (A) fins of constant cross-sectional area, (B) fins of constant cross-sectional area gradient, and (C) radial fins with power-law cross-sectional area distribution. The types (A) and (B) fins are allowed having variable power-law profile distributions. The energy equation for each case is solved analytically, and closed form equations for various performance indicators including the fin efficiency are obtained. It is identified that high performance fins are those fins having larger tip and surface areas and smaller tip thickness than those of the straight fin. These effects tend to augment the fin heat transfer rate. One of the indicators of the type (A) fins is found to become dependent on the tip thickness when it is smaller than the base thickness and for thermal lengths larger than 1.5. The type (C) fins are found to transfer more thermal energy than the type (B) fins under same tip area and tip thickness. Finally, generalized correlations for the fin efficiency of high performance fins as function of base and tip areas and thicknesses and the thermal length are proposed and validated.

Heat transfer inside wall-joint-fins systems is analyzed. The coupled two-dimensional energy equations of the wall and the joint-fin are solved numerically using an iterative high order scheme finite volume method. Advanced fine analytical solution is proposed and various closed form equations for different heat transfer augmentation indicators are obtained. Excellent agreement is noticed between the numerical and the analytical results. Wall-joint-fins systems are more effective in transferring thermal energy when the joint-fin is made of a highly conductive material. Moreover, varying the joint-fin lengths ratio may increase the system effectiveness by a factor of 1.2. In addition, the maximum reported system effectiveness is 925% above that when both wall and joint-fin have same thermal conductivity. The maximum system effectiveness which occurs at specific geometrical aspect factors increases as convective heat transfer coefficients increase. Furthermore, the effectiveness and efficiency of the wall-joint-fins system increase as the relative joint-fins to wall volume ratio increases. The wall-joint-fins efficiency is least affected by the joint-fin lengths ratio. Eventually, the heat transfer coefficient between the joint-fin and the wall is identified. Finally, wall-joint-fin systems are recommended as heat transfer enhancing elements.

Enhancement of heat transfer in minichannels due to co-flowing of two immiscible fluids in a direct contact is investigated in this work. Different fluid combinations are analyzed. The momentum and energy equations for both flows are solved analytically and numerically. The numerical and analytical solutions are found to be in good agreement. A parametric study including the influence of fluids relative viscosity, thermal conductivity, thermal capacity and height ratios is conducted for various Peclet numbers. Different ranges of the parameters that augment the heat transfer are obtained, and different physical aspects of the problem are discussed. For practical fluid combinations with small Peclet numbers, the enhancement factor can increase up to 2.6 folds. However, that increase is about 1.2 folds when the Peclet number is increased by two orders of magnitude. This work establishes the mechanisms for heat transfer enhancement utilizing two immiscible co-flows.

Heat transfer through a square cavity enclosing a thin hollow-cylinder is theoretically investigated in the present work. The coupled continuity, momentum and energy equations governing the fluids contained in the present system are solved numerically. High accurate finite volume method utilizing well-known CFD software is implemented. Good agreement was obtained between the numerical and published results under special constraints. It is found that a significant improvement in the insulating property of the present system as compared to that of pure square cavity is attainable when the hollow-cylinder is placed very close to the middle of the isothermal boundaries. Moreover, larger hollow cylinder diameters and smaller Grashof numbers are found to produce further improvement in the insulating property of the present system. In addition, the maximum reported enhancement in the insulating property of the present system is 1.96 times that of the pure closed square cavity, The reported optimum management is equivalent to having a square cavity filled with a number of solid rods equals to 4.0 times that producing the same insulating property of the pure square cavity. Accordingly, a building envelope made of at least bricks with horizontal square perforations assembled through an at least single layered network of horizontal hollow-pipes placed close to the middle of hot/cold boundaries is recommended. (C) 2012 Elsevier B.V. All rights reserved.

Heat transfer inside permeable hollow-fin systems is analyzed in this work. Two types are considered: (A) the permeable hollow-pin, and (B) the permeable hollow joint-pin. The governing partial differential equations are solved numerically using a well-known implicit, iterative and finite-difference method. The numerical solutions are validated against various analytical solutions derived based on different constraints. It is found that the permeable hollow-pin can transfer more thermal energy than the solid pin when an external suction flow is present at the outer surfaces. Moreover, the maximum reported heat transfer rate due to permeable hollow-pin is 362 percent above that of solid pin at dimensionless suction flow number equals to 3.0. Furthermore, the maximum reported heat transfer rate due to permeable hollow joint-pin is 44 percent above that of the solid joint-pin at dimensionless suction flow number equals to 2.0. In addition, the permeable hollow joint-pin is found to be capable of transferring more thermal energy than the solid joint-pin at a specific joint-pin lengths ratio depending on the values of the various controlling parameters. Finally, this work demonstrates that by using combined heat transfer enhancement approaches, novel heat transfer enhancers can be proposed.

PurposeThe purpose of this work is to consider heat transfer inside wall‐rooted‐fins systems.Design/methodology/approachThe coupled two‐dimensional heat diffusion equations are nondimensionlaized and solved numerically using an iterative finite volume method. Approximate fine analytical solutions for various augmentation indicators are derived. Excellent agreement is obtained between the numerical and the analytical results. A parametric study including all of the involved dimensionless parameters is conducted and presented graphically. Accurate correlations are generated.FindingsIt is found that fin‐roots with large root lengths experience bi‐directional heat transfer rates. Moreover, the wall‐rooted‐fins system is found to possess an effectiveness that can be more than 60 percent above that with rootless fins at wall Biot numbers of unity order. This value is found to increase as the Biot number increases or as the wall‐to‐fin thermal conductivities and volumes ratios decrease. In addition, heat transfer rates through wall‐rooted‐fins systems can be more than 100 percent above those having uniform thermal conductivities. Eventually, the heat transfer coefficient between the fin‐roots and wall are derived, which is found to be independent on the wall thickness.Originality/valueFinally, this work paves a way for an effective passive method for augmenting heat transfer inside wall‐fins systems.

121104-9898-IJMME-IJENS © August 2012 IJENS I J E N S Abstract-In the present study, a numerical model has been developed to investigate thermal performance and optimization of relevant thermogeometric parameters involved in a convective cooling system. The heat removal system comprises of flow of fluid or nanofluid through a rectangular channel with its lower surface subjected to constant heat flux and the upper surface equipped with pin fins having variable distributions. The governing energy equation and momentum equation have been solved to find an optimum profile for variable fin density. Comparison of Nusselt numbers for non-uniform fin distribution with those for uniformly distributed fins reveals 2.7% improvement in heat transfer rate for the former case. This increase was associated with Peclet umber of Pe=1.5, No=0.9, Nf=0.001 and θα= –0.5 for a variable fin density profile with B=0.045 and Φo=0.6. Furthermore, two nanofluids one containing copper oxide nanoparticles dispersed in water and the other in a base fluid of 60:40 (by mass) ethylene glycol and water mixture (60:40 EG/W) have been used in present computations with particle volumetric concentration ranging from 0 to 6%. Since heat flux is provided at the lower surface of the channel, Nusselt numbers were calculated only at the lower surface. The analyses reveal about 65% increase in average Nusselt number at low Peclet number (Pe=1.0) and about 27% increase in that at high Peclet number (Pe=200). Moreover, Effort has also been made to explore the possibility of using some published dispersed model for nanofluids to fit a published experimental thermal conductivity model of nanofluids. It is shown that both models can be excellently matched based on equal local or average Nusselt numbers at specific dispersion model constant coefficients. These coefficients are mainly depending on nanoparticles material, particles volumetric concentration and Peclet number. Finally, a correlation for the dispersion model coefficients as functions of the relevant parameters has also been proposed.

The present work analyzes theoretically and verifies the advantage of utilizing ɛ-microcantilever assemblies in microsensing applications. The deflection profile of these innovative ɛ-assembly microcantilevers is compared with that of the rectangular microcantilever and modified triangular microcantlever. Various force-loading conditions are considered. The theorem of linear elasticity for thin beams is used to obtain the deflections. The obtained defections are validated against an accurate numerical solution utilizing finite element method with maximum deviation less than 10 percent. It is found that the ɛ-assembly produces larger deflections than the rectangular microcantilever under the same base surface stress and same extension length. In addition, the ɛ-microcantilever assembly is found to produce larger deflection than the modified triangular microcantilever. This deflection enhancement is found to increase as the ɛ-assembly's free length decreases for various types of force loading conditions. Consequently, the ɛ-microcantilever is shown to be superior in microsensing applications as it provides favorable high detection capability with a reduced susceptibility to external noises. Finally, this work paves a way for experimentally testing the ɛ-assembly to show whether detective potential of microsensors can be increased.