Experimental investigation of heat transfer enhancement using impingement of multiple water jets

The problem of cooling electronic components has become a subject of special interest in recent years due to the increasing capacity and rapidly decreasing size of electronic components. Direct contact cooling using multiple jet impingement is considered the most effective method. The heat transfer problem is complex and a better understanding of the jet impingement method is essential for the proper application of this method for electronic cooling. Investigations were carried out using an electrically heated test plate. Heat flux in the range of 25 to $200 \ \hbox{W/cm}^{2}$ , which is a typical requirement for cooling high power electronic components was dissipated using 0.5‐mm diameter water jets arranged in a 7×7 array with a pitch of 3 mm. Temperature difference between the test plate and water was within $30 \ ^{\circ}\hbox{C}$ . Tests were performed in the flow rate range of 22 to 40 ml/min, resulting in a Reynolds number range of 1100 to 1750. Results show a significant increase in the heat transfer coefficient or Nusselt number with an increase in heat flux. The effect of the flow rate or Reynolds number on the heat transfer coefficient is found to be negligible. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20291     

Unsteady MHD mixed convection flow of a micropolar fluid along an inclined stretching plate

In this research, the unsteady magnetohydrodynamic mixed convection flow of a micropolar fluid over an inclined plate has been investigated. The problem is reduced to a system of non‐dimensional partial differential equations, which are solved numerically using the implicit finite‐difference scheme. Velocity profiles, temperature profiles, concentration profiles, the skin friction coefficient, the rate of heat transfer, and the rate of mass transfer are computed numerically for various values of different physical parameters. In this study, we consider both assisting and opposing flow. It is found that in the assisting flow case, a solution could be obtained for all positive values of the buoyancy parameter λ, while in the opposing flow case the solution terminated at $\lambda = {\lambda _c}(\lambda < 0)$ . © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21034     

Numerical and experimental study of free convection through a horizontal open‐ended axisymmetric cavity

This paper summarizes a numerical and experimental investigation of free convective heat transfer in an open‐ended cavity between two horizontal parallel circular plates. The upper plate is maintained at an ambient temperature and the lower one is heated. Air is used as the heat transfer medium. The numerical model equations are solved using a control volume‐based finite differences method, and the experimental study was performed using holographic interferometry. Streamlines and isotherm patterns are presented and discussed for different aspect ratios (A) and Rayleigh numbers (Ra). Heat transfer at the surface of the lower plate is thoroughly inspected in the ranges and . Useful correlations of Nusselt numbers in terms of and A are given with their validity ranges. Also, an investigation of both numerical and experimental results is performed. It shows similar temperature field aspect with some differences in the radial boundary layer thickness and a small deviation in the heat transfer.     

Exact,analytic temperature distributions of pin fins with constant thermal conductivity and power‐law type heat transfer coefficient

In this Technical Note, the problem of determining the temperature distribution in a pin fin with power‐law heat transfer coefficients is addressed. It is demonstrated that the governing fin equation, a nonlinear second‐order differential equation, is exactly solvable for the entire range of the exponent n in the power‐law heat transfer coefficients. The exact, closed‐form analytical solutions in implicit form are convenient for physical interpretation and optimization for maximum heat transfer. Furthermore, it is proved that the exact solutions have three different structures: (1) dual in the range of , (2) unique or dual in the range of , and (3) unique in the range of . Additionally, exact analytical expressions for the fin efficiency and the fin effectiveness are provided, both as a function of the dimensionless fin parameter for the gamma of n under study.     

Numerical simulation of mixed convection in a porous medium filled with water/Al2 O3 nanofluid

The present work encloses the application of a Brinkman–extended Darcy model in a problem concerning mixed convection ina lid–driven porous cavity using nanofluids. The transport equations are solved numerically by the finite volume method on a co–located grid arrangement using the Quadratic Upstream Interpolation for Convective Kinematics (QUICK) scheme. The effects of governing parameters, namely, Grashof number (Gr), Darcy number (Da), and solid volume fraction $(\chi )$ , on the streamlines and the isotherms are studied. The present results are validated by favorable comparisons with previously published results and are in good agreement with them. The present numerical results show that the addition of nanoparticles to a base fluid has produced an augmentation of the heat transfer coefficient and it is found to increase significantly with an increase of the particle volume concentration. It is observed from the results that at the higher value of the Grashof number (Gr = 10⁴ ), the average Nusselt number increases with an increase in the Darcy number for a constant solid volume fraction. The detailed results are reported by means of streamlines, isotherms, and Nusselt numbers. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21029     

Numerical investigation of the effects of geometric parameters of heaters on mixed covection in a lid‐driven cavity filled with different nanofluids

In this paper, the effects of the thicknesses and locations of two rectangular heaters, located on the bottom and one side of on an enclosure, on mixed convection of nanofluid flows in a lid‐driven cavity are numerically investigated. The enclosure is simultaneously heated partially by these two heaters which have similar or different thicknesses and also filled with different nanofluids containing nanoparticles of Cu, Ag, Al₂O₃, and TiO₂ within the base fluid of water. A finite volume approach by the SIMPLE algorithm is used to solve the governing equations. The effects of different Rayleigh numbers (), Reynolds numbers (), solid volume fractions (), heater lengths (), heater locations () and heater thicknesses () on the streamlines, isotherms and the average Nusselt number along two heaters are studied accurately. Also, variations of average Nusselt number of two heaters are considered whenever one heater is fixed and the other heater moves along on the wall. Moreover, variations of the length of one heater on the average Nusselt number are also studied whenever the length of the other heater is fixed. In addition, variations of the thickness of one heater on the average Nusselt number are studied whenever the thickness of the other heater is fixed.     

Mixed convection inside a fluid‐porous composite cavity with centrally rotating cylinder

The mixed convection in a fluid‐porous composite medium lying inside a square cavity with a centrally rotating cylinder has been investigated in the present work. The bottom half of the cavity is filled with a porous material and the top half is filled with a clear fluid. The bottom wall of the cavity is at a higher temperature, and the top wall is at a lower temperature. The vertical walls are thermally insulated. The convection inside the cavity sets through the combined mechanisms of the thermal buoyancy force and the shearing action of the centrally rotating cylinder. The relative importance of each driving mechanism over the other is featured through the Richardson number. The Darcy–Brinkman–Forchheimer equation is used for the flow modeling in the porous medium, and a single‐domain approach is adopted for the numerical solution in the fluid‐porous composite medium. The simulation is carried out with ANSYS Fluent software, and a parametric analysis involving the Rayleigh number (), Richardson number (), and the Darcy number () is conducted showing their effects on the flow and heat transfer. The phenomena are quite interesting at higher Darcy number and Rayleigh number. The distributions of isotherms, streamlines, and vector plots are plotted, along with the local Nusselt numbers for different parameters, to explore the underlying physics of the phenomenon. The system is found stable at lower Darcy number, and the heat transfer is minimum around Ri = 10. From the numerical study, an empirical correlation for the average Nusselt number is developed as a function of the other dimensionless numbers.     

Modeling and Optimization of Nanofluid Flow in Flat Tubes Using a Combination of CFD and Response Surface Methodology

In this paper, modeling and optimization of Al₂O₃–water nanofluid flow in horizontal flat tubes is performed using a combination of computational fluid dynamics (CFD) and response surface methodology (RSM). At first, nanofluid flow is solved numerically in various flat tubes using CFD techniques and the heat transfer coefficient () and pressure drop () in tubes are calculated. The numerical simulations are performed using two phase mixture model by FORTRAN programming language. The flow regime and the wall boundary conditions are assumed to be laminar and constant heat flux respectively. In the second step, numerical data of the previous step will be used for a parametric study, modeling and optimization of nanofluid flow in flat tubes using the RSM technique.It is shown that the results include important design information on nanofluid parameters in flat tubes. The important design information about the relationship between design variables and responses will not be achieved without the simultaneous use of CFD and optimization approaches.     

MHD natural convection inside an inclined trapezoidal porous enclosure with internal heat generation or absorption subjected to isoflux heating

A steady laminar two‐dimensional magneto‐hydrodynamic natural convection flow in an inclined trapezoidal enclosure filled with a fluid‐saturated porous medium is investigated numerically using a finite difference method. The left and right vertical sidewalls of the trapezoidal enclosure are maintained at a cold temperature. The horizontal top wall is considered adiabatic while the bottom wall is subjected to isoflux heating. A volumetric internal heat generation or absorption is embedded inside the trapezoidal enclosure while an external magnetic field is applied on the left sidewall of the enclosure. In the current work, the following parametric ranges of the non‐dimensional groups are used: Hartmann number is varied as , Darcy number is taken as , 10^?4, and 8 × 10^?5, Rayleigh number is varied as , Prandtl number is considered constant at Pr = 0.7, the dimensionless internal heat generation or absorption parameter is varied as Δ = ?0.2, 0, 1, and 2.0, while the trapezoidal enclosure inclination angle is varied as . The results indicated a strong flow circulation occurs when the Darcy and the Rayleigh numbers are high. In addition, it is found that the Hartmann number, internal heat generation or absorption parameter and inclination angle have an important role on the flow and thermal characteristics. It is also found that when the enclosure inclination angle and Hartmann number increase the average Nusselt number along the hot bottom wall decreases. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21013     

Energy and Exergy Analysis of a Single‐Pass Sequenced Array Baffled Solar Air Heater with Packed Bed Latent Storage Unit for Nocturnal Use

The current study presents an experimental investigation on evaluation of thermal performance of a single‐pass double‐glazed solar air heater with the use of packed bed paraffin wax as a phase change material (PCM). Moreover, the absorber plate is equipped with baffles attached over its top. Galvanized sheets with a thickness of 0.4 mm and total surface areas of 30 cm² are chosen as baffles that are placed in a sequential manner over the absorber plate. The solar energy was stored in the packed bed PCM during the diurnal period (charging process) and was released at night for nocturnal use (discharging process). The tests were performed at three different mass flow rates of 0.009 0.014 and 0.017 resulting in the creation of different Reynolds numbers along the channel. The measured parameters were inlet, outlet, and the PCM temperatures under the meteorological condition of Mashhad, Iran. Energy and exergy efficiencies of the system have been calculated according to the first and second laws of thermodynamics. The experimental results illustrate that the daily energy efficiency varied between 20.7% and 26.8%, whereas the daily exergy efficiency varied between 10.7% and 19.5%.     

Influence of chemically reactive species and a volumetric heat source or sink on mixed convection over an exponentially decreasing mainstream

In this paper, we investigate mixed convection flow over an exponentially decreasing freestream velocity in presence of nonlinear chemically reactive species and a volumetric heat source or sink. Nonsimilar transformations are used to reduce the boundary layer equations into dimensionless equations and are further solved by the implicit finite difference scheme in combination with the quasi‐linearization technique. The influence of various governing parameters such as the volumetric heat source/sink parameter (Q), the ratio of buoyancy forces (N), the Richardson number (Ri), and the chemical reaction parameter (Δ) on the flow, thermal and species concentration fields are discussed and presented in terms of graphs. The numerical investigation reveals that the increase in volumetric heat source/sink parameter Q increases the temperature profile about 69% in presence of injection and the concentration profile decreases about 56% for and increases around 53% for as n increases from 1 to 2.     

Double Diffusive Flows over a Stretching Sheet of Variable Thickness with or without Surface Mass Transfer

This paper presents a numerical investigation of the steady two‐dimensional mixed convection flow along a vertical semi‐infinite stretching sheet of variable thickness. The effect of double diffusion on velocity, thermal and concentration fields in presence of power‐law temperature and concentration distributions at wall along with surface mass transfer is considered. The nonlinear coupled partial differential equations governing the flow, thermal and concentration fields are first transformed into a nondimensional set of coupled nonlinear partial differential equations and solved numerically using an implicit finite‐difference scheme in combination with the Newton's linearization technique to obtain nonsimilar solutions at each stream‐wise location. Numerical results are presented to discuss the effects of various physical parameters on the velocity, temperature, and concentration fields. Furthermore, the numerical results for the local skin friction coefficient, local Nusselt number, and local Sherwood number are also reported. For a fixed buoyancy force, the skin friction coefficient and Nusselt number increase with Prandtl number. The increase in the Prandtl number causes about a 30% reduction in the thickness of the thermal boundary layer. The wall thickness parameter enhances the thickness of the momentum boundary layer and the velocity overshoot is observed up to 20% for wall thickness parameter . In contrast, the increase of power‐law index parameter m from to reduces approximately 10% to 25% the momentum and thermal boundary layer thicknesses depending on the values of other parameters     

Thermal analysis on a groove‐enhanced minichannel application

A grooved surface feature is considered as a thermal enhancement for electronics cooling with single‐phase flow in minichannels. The influence of the groove structure and groove depth‐to‐width ratio on the flow and heat transfer in the minichannels has been numerically investigated. A two‐dimensional turbulent flow model was employed to optimize the groove structure in the minichannel. The effect of the groove geometric shape on the heat transfer performance in the minichannel was analyzed by evaluating the fluid thermophysical parameter and Nusselt number. It is found that the average Nusselt number ratio ($Nu_{avg}^{*}$ ) changes with an increase in groove depth‐to‐width ratios from d/W = 0.1 to d/W = 0.5. The groove heat transfer unit number (NTU) integrating heat transfer area (A) and heat transfer coefficient (h) were defined. The change of the NTU^* for the minichannel with a triangular groove is different from that of the minichannel with a cylindrical groove while the groove depth‐to‐width ratio varies from d/W = 0.1 to d/W = 0.5. In addition, the flow pressure loss across the groove and the effects of the Reynolds number in the minichannels were also investigated. All the results should be taken into account for a better design of a minichannel with groove. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20413     

Experimental Investigation of Heat Transfer Characteristics and Cooling Performance of Mini‐ and Microheat Exchangers

This paper presents an experimental investigation of forced convection heat transfer in two heat sinks for electronic system cooling and investigated the comparisons of the thermal behavior of the mini‐ and microchannel heat exchangers. The hydraulic dimension of one of the heat sinks is 2 mm while that of the other is . Deionized water was used as the working fluid for studies conducted in both the heat exchangers. The effect of heat flux and volumetric flow rate (in laminar flow regime) on temperature and heat transfer coefficient is studied. Irrespective of the average heat transfer coefficient and the total thermal resistance, advantages and limitations of each device are analyzed and discussed in the light of experimental results. Furthermore, the results obtained from the experiments were in good agreement with those obtained from the design theory analyses.     

Forced Convection Heat Transfer of Newtonian Fluids from a Backward‐Facing Step: Effects of Expansion Ratio,Reynolds, and Prandtl Numbers

This study involves the numerical solution of the laminar heat transfer in a separating and reattaching flow by simulating the flow and heat transfer downstream of a backward‐facing step. The in‐house finite volume code has been implemented employing a hybrid differencing scheme and the SIMPLE algorithm for the pressure–velocity coupling. Three principal parameters governing heat transfer in this geometry, that is channel expansion ratio (ER), Reynolds number (Re), and Prandtl number (Pr), are systematically varied in the range ER = 1.111 to 2, Re = 1 to 200, and Pr = 0.71 to 100, and the simple correlations between these parameters have been elucidated. A series of important findings have been established by analyzing the results some of which are: (1) there is an associated shifting of the point of maximum heat transfer with respect to the flow‐reattachment point with gradually decreasing the values of ER and (2) the heat transfer enhancement increases with the increase in Pr number as a result of the compression of the thermal boundary layer and the maximum Nusselt number varies as .     

Aerodynamic optimization of wind turbine rotors using a blade element momentum method with corrections for wake rotation and expansion

The blade element momentum (BEM) method is widely used for calculating the quasi‐steady aerodynamics of horizontal axis wind turbines. Recently, the BEM method has been expanded to include corrections for wake expansion and the pressure due to wake rotation (), and more accurate solutions can now be obtained in the blade root and tip sections. It is expected that this will lead to small changes in optimum blade designs. In this work, has been implemented, and the spanwise load distribution has been optimized to find the highest possible power production. For comparison, optimizations have been carried out using BEM as well. Validation of shows good agreement with the flow calculated using an advanced actuator disk method. The maximum power was found at a tip speed ratio of 7 using , and this is lower than the optimum tip speed ratio of 8 found for BEM. The difference is primarily caused by the positive effect of wake rotation, which locally causes the efficiency to exceed the Betz limit. Wake expansion has a negative effect, which is most important at high tip speed ratios. It was further found that by using , it is possible to obtain a 5% reduction in flap bending moment when compared with BEM. In short, allows fast aerodynamic calculations and optimizations with a much higher degree of accuracy than the traditional BEM model. Copyright © 2011 John Wiley & Sons, Ltd.     

Evaluation method of the energy conversion efficiency of coal gasification and related applications

Traditional gasification parameters, such as cold gas efficiency, hot gas efficiency, or thermal efficiency, only evaluate the heat energy utilisation efficiency of gasifiers and do not take into account the gasification processes expending electricity and other types of energies. Therefore, the energy conversion efficiency cannot be assessed using these parameters. The calculation process on the energy conversion efficiency of underground coal gasification (UCG) is the basis for obtaining quantitative data of carbon emission reduction and establishing the carbon trading methodology of UCG. Moreover, the energy conversion efficiency both for surface coal gasification and UCG is a key research topic because it directly affects the economic and environmental benefits of gasification projects. This study proposed that two parameters, the integrated gasification efficiency (h_com) and the hot gas integrated gasification efficiency ( ), should be included into the coal gasification parameters and used to evaluate the energy conversion efficiency of coal gasification. In addition, the calculation methods of these two parameters for both surface gasification and UCG were established. Using the method, h_com and , of the UCG and Texaco gasification under the same scale was compared and that of various UCG processes was calculated. The results proved the necessity and reasonability of the two parameters and suggested that a certain amount of CO₂ was favourable to improve h_com and of UCG. However, a certain amount of pure O₂ can improve h_com of UCG without direct influences on . Under the condition of each process, to maximise h_com and , there must be an optimal steam (CO₂) to O₂ rate. Copyright © 2015 John Wiley & Sons, Ltd.     

Free Convective Flow of Pseudo‐Plastic and Newtonian Fluid Past a Convectively Heated Vertical Plate in a Darcian Porous Medium with Heat Generation/Absorption

We study the effect of thermal convective boundary condition and yield stress on free convection heat transfer for a pseudo‐plastic and Newtonian fluid past a permeable vertical flat plate which is embedded in a Darcian porous medium in the presence of heat generation/absorption numerically. Instead of using similarity transformations available in the literature, we have developed them by one point transformation and hence transform the governing boundary layer equations into corresponding similarity equations. The resulting similarity equations were solved using Runge–Kutta–Fehlberg fourth fifth (RKF45) order numerical method. The effect of the governing parameters, namely the power index of pseudo‐plastic fluids n, the rheological parameter Ω, heat generation/absorption parameter Q, suction/injection parameter , and the convective heat parameter B on the dimensionless velocity, the temperature and the heat transfer rates were investigated. A close agreement is found between our results and published results. Our present study finds application in printing and polymer industries and fluid phenomena associated with concentrated suspensions.     

Thermal Analysis of Nanofluids on the Thin Film Evaporation of Meniscus

A mathematical model for predicting evaporation in the thin film region was developed and its analytical solutions were obtained for thin‐film thickness, the heat transport per unit length and the total heat flux transport in the thin‐film region. These analytical solutions show that the higher heat flux through the thin film region occurs due to the higher superheat. The maximum evaporative rate occurs when the effects of the increase in the temperature difference and in the thin film thickness on the heat flux q stay equal. A nanofluid, which is a colloidal mixture of nanoparticles (1 nm to 100 nm) and a base liquid (nanoparticle fluid suspensions), is employed as the working fluid. In a certain range, increasing the volume fraction of nanoparticles in the base fluid leads to decreasing the kinematic viscosity of the nanofluid and increasing the thermal conductivity, which influences the evaporation in the thin film region. The heat transfer rate per unit length and the total heat flux in the thin film region display various characteristics among the different type of nanofluids due to the differences of the kinematic viscosity and the thermal conductivity.     

Heat Transfer Control of an Impinging Inclined Slot Jets on a Moving Wall

This work is devoted to the numerical study of the interaction of an inclined plane turbulent jet with a moving horizontal isothermal hot wall. The inclination of the jet allows the control of the stagnation point location. The numerical predictions based on statistical modeling are achieved using second order Reynolds stress turbulence model coupled to the enhanced wall treatment. The jet Reynolds number (Re), surface‐to‐jet velocity ratio (R_sj); and optimal inclination angle of the jet (α) are varied. The calculations are in good agreement with the available data. The numerical results show that the heat transfer is greatly influenced by the jet Re and the velocity of the moving wall. The local Nusselt number (Nu) decreases with increasing R_sj (until R_sj = 1). However, the optimal inclination of the jet enhances heat transfer and modifies significantly the stagnation point location. Average Nu is correlated according with the problem parameters as .