Uniform Heat Flux Boundary Research Articles

Experimental analysis of heat loss of a non – evacuated parabolic trough collector receiver subjected to uniform and non-uniform wall heat flux condition

ABSTRACT The parameters that largely influence the design and performance of a parabolic trough collector system for delivering process heat are optical efficiency, incidence angle modifier, solar irradiation data, and receiver heat loss. The heat loss and optical properties of a receiver greatly influence the overall efficiency of the solar thermal system. This paper focuses the attention on the experimental investigation of heat loss associated with a non-evacuated receiver subjected to i). Uniform wall heat flux (UWHF) and ii). Non – uniform wall heat flux (NUWHF) boundary condition. A series of heat-loss measurements are carried out in the novel test stand capable of emulating both the boundary conditions, on the indigenous non – evacuated receiver developed for moderate temperature process heat applications. A comparative analysis of natural and forced convection heat loss associated with the bare and glass-covered receiver is performed. The results show that the non – uniform wall heat flux boundary condition has a decisive influence over the forced convection heat loss, especially at higher wind velocities. The work concludes that i) the surface temperature of the receiver associated with natural convection heat loss and subjected to NUWHF boundary condition is1.5– 5% higher than the receiver subjected to UWHF boundary condition. ii) In forced convection mode of heat loss the surface temperature of the receiver is 4–13% higher than the receiver subjected to UWHF. Thus, a more realistic characterization of the thermal behaviour of a non-evacuated receiver for moderate temperature process heat application is possible only by investigating the receiver subjected to non – uniform heat flux distribution. Such a characterization of the receiver will considerably influence the aperture area and thus the cost of the collector to deliver process heat.

Evaluation of candidate strategies for the estimation of local heat transfer coefficient from wall jets

ABSTRACTThis paper reports results of experimental investigations on planar and three-dimensional wall jets over a flat surface. The local heat transfer coefficient is estimated at transient conditions with a semi-infinite approximation and at steady state conditions with a uniform wall heat flux boundary. Liquid crystal thermography and infrared thermography are used to map the surface temperatures. Experiments are performed for 2000 Re 8000 and 0 x/L 80. Results show that transient infrared thermography with semi-infinite approximation is a better candidate for the estimation of the heat transfer coefficient from wall jets.

Numerical Analysis of Conjugated Convection-Conduction Heat Transfer in a Microtube Gas Flow

Abstract Numerical solutions for conjugate heat transfer of a hydro-dynamically fully developed, thermally developing, steady, incompressible laminar gas flow in a microtube with uniform wall heat flux boundary condition are presented. The mathematical model takes into account effects of rarefaction, viscous dissipation, flow work, shear work, and axial conduction in both the wall and the fluid. The effect of the tube wall thickness, the wall-to-fluid thermal conductivity ratio, as well as other factors on heat transfer parameters is investigated, and comparisons with the case of zero wall thickness are presented as appropriate. The results illustrate the significance of heat conduction in the tube wall on convective heat transfer and disclose the significant deviation from those with no conjugated effects. Increasing the wall thickness lowers the local Nusselt number. Increasing the wall-to-fluid thermal conductivity ratio also results in lower Nusselt number. In relatively long and thick microtubes with high wall-to-fluid thermal conductivity ratio, the local Nusselt number exhibits minimum values in the entrance regions and at the end sections due to axial conduction effects. The analysis presented also demonstrate the significance of rarefaction, shear work, axial conduction, as well as the combined viscous dissipation and flow work effects on heat transfer parameters in a microtube gas flow. The combined flow work and viscous dissipation effects on heat transfer parameters are significant and result in a reduction in the Nusselt number. The shear work lowers the Nusselt number when heat is added to the fluid.

The Reasons of Heat Transfer Enhancement in a Laminar Channel Flow With Uniform Heat Flux on the Wall Under a Pair of Longitudinal Vortex Generators Mounted on the Bottom Wall

To find the reasons of heat transfer enhancement of a laminar convective heat transfer process in a channel at a uniform heat flux boundary when a pair of longitudinal vortex generators (VGs) is mounted on the bottom wall, the laminar convective heat transfer process in the channel is investigated numerically in a frame built up by the convective transport equation of the heat flux. The results show that longitudinal vortices greatly increase the local convection contribution terms that determine the local intensity of the convective transport of the heat flux component in the span direction, and that the increased local contribution terms intensify the local convective transport of the heat flux component in the same direction. This process increases the convection contribution terms that determine the convective transports of the heat flux components in the main stream direction and in the normal direction of the channel walls. The increase in these convection contribution terms results in an enhancement of the convective heat transfer ability on the channel walls, and then, the heat transfer is enhanced by longitudinal vortices. When the span-averaged characteristic is numerically counted, longitudinal vortices are found to have no contribution on the span-averaged convective transport of the heat flux in the span direction.

Three-Dimensional Turbulent Flow, Heat Transfer and Second-Law Analysis in a Circular Duct with Single Baffle

Three-dimensional turbulent-forced convection flow, heat transfer and second-law analysis in a circular duct having single baffle have been examined numerically under uniform constant wall heat flux boundary condition at steady state. Baffle is attached in the entrance, middle and exit regions of the test section. ANSYS Fluent 15 which uses finite-volume method has been employed for numerical analysis. The effects of Reynolds number Re changing from 3000 to 50,000 and dimensionless position of baffle S/D = 1, 16.1 and 25 are investigated for Prandtl number of Pr = 0.7 and baffle angle of α = 90°. It is seen that circular duct with single baffle has a higher Nusselt number, friction factor and entropy generation rate compared to the circular duct without baffle. It is also seen that the duct with baffle in the inlet region has a higher value of Nusselt number and friction factor. The duct having baffle in the middle region has a maximum thermal performance and low entropy generation rate. The accuracy of the results is validated by comparing the obtained results with the results of smooth duct.

Experimental investigation on convection heat transfer characteristics of flat plate under environmental wind condition

ABSTRACTBy establishing wind tunnel and employing electrical heating method, the heat transfer characteristics of flat plate were investigated under environmental wind condition. Both the uniform and linear heat flux boundary conditions were adopted for comparison purpose. Besides, the impacts of heat flux qwm, tilt angle α and wind incidence angle θ on heat transfer were explored in cases of windward and leeward facing positions. The local convection heat transfer coefficient hcx and average convection Nusselt number Nucm were obtained. The results show that, when α is small, there are two maximum values of hcx under linear heat flux boundary condition. As for Nucm, differences between the two boundary conditions seem indistinctive. At windward orientation, Nucm is not sensitive to α. While for leeward orientation, Nucm has a rapid decline progress with the increasing α, which indicates the tilt angle α is an important parameter to affect the heat transfer of plate. Finally, new correlations of Nucm have been developed, which were proven effective in engineering applications.

The Consequence of Target Surface Curvature in the Jet Impingement Cooling

An experiment sturdy has been carried out for jet impingement cooling on the spherically convex surface is the development of mechanism. The effect of curvature, Space between jet exit and target surface, and Reynolds number on heat transfer is investigated for around air jet on hemispherical surface. The flow at the jet exit has fully developed velocity profile. A uniform heat flux boundary is created on the heated surface. The experiments are performed for 5000<Re<25000, 2<L/d<10, and jet diameters ranging from 1.3, 2.1, 3.4, 4.0 and 5.2 cm. In the mean time effect of curvature on local heat transfer is negligible at the wall jet region corresponding to r/d>0.5. From the experimental results the variation of the D/d ratio with local Nusselt number (Nust) for various Reynolds numbers and various L/d ratios are plotted. The results show that Nust increase with increase in curvature and the effect of the curvature will high at high Reynolds number. i.e. Nust at Re=25000 is 25% higher than at Re= 5000 This may be attributed to an increase in curvature increases acceleration, & size of three dimensional counter rotating vortices at stagnation point and the increment of Reynolds number increases the jet momentum, and also enhances the vortices creation. Nust is peaking in the L/d ratio of 6 because of high turbulence intensity as this distance.

Laminar Natural Convection Between a Vertical Surface With Uniform Heat Flux and Pseudoplastic and Dilatant Fluids

This paper reports the average Nusselt number for steady, laminar natural convection between a vertical surface and otherwise quiescent pseudoplastic and dilatant fluids under a constant and uniform surface heat flux boundary condition. Models for the fluids' apparent viscosity were utilized that are valid in all five regions of the flow curve. The results are thus applicable for whatever shear rates may exist within the flow field and a dimensionless shear rate parameter was identified that quantifies the shear rate region where the given system is operating. The data indicate that the average Nusselt numbers approach the corresponding Newtonian values when the shear rates are predominantly in either the zero or the infinite shear rate Newtonian regions. However, power law values are approached only when both of the following two conditions are met: (1) the shear rates are principally in the power law region and (2) the fluid's limiting zero and infinite shear rate Newtonian viscosities differ sufficiently, by approximately 4 orders of magnitude or more. For all other cases, the average Nusselt number was found to reside between the Newtonian and the power law asymptotes. Results are provided in both graphical and tabular form over a broad range of system parameters.

Numerical study of turbulent flow and heat transfer of Al2O3–water mixture in a square duct with uniform heat flux

Turbulent flow and convective heat transfer of a nanofluid made of Al2O3 (1–4 vol.%) and water through a square duct is numerically studied. Single-phase model, volumetric concentration, temperature-dependent physical properties, uniform wall heat flux boundary condition and Renormalization Group Theory k-e turbulent model are used in the computational analysis. A comparison of the results with the previous experimental and numerical data revealed 8.3 and 10.2 % mean deviations, respectively. Numerical results illustrated that Nu number is directly proportional with Re number and volumetric concentration. For a given Re number, increasing the volumetric concentration of nanoparticles does not have significant effect on the dimensionless velocity contours. At a constant dimensionless temperature, increasing the particle volume concentration increases the size of the temperature profile. Maximum value of dimensionless temperature increases with increasing x/Dh value for a given Re number and volumetric concentration.

Heat Transfer Measurements for the Horizontal Micro-Tubes Using Liquid Crystal Thermography

In this study, a non-contact liquid crystal thermography (LCT) method was developed for surface temperature measurements on the micro- and micro-tubes with the diameters of 2000µm, 1000µm, 838µm and 686µm. In the single-phase heat transfer experiments, the fully-developed flow heat transfer data from laminar to turbulent regionswere obtained under the uniform wall heat flux boundary condition. The experiment system was verified with the macro-tube of 2000µm diameter tube. The results showed that buoyancy effect was not observed in the laminar region when the tube diameter was less than 1000 µm. In theturbulent region, the heat transfer data trend followed with the classical turbulent correlation. Furthermore, the current LCT results showed the similar trend when compare with the published LCT data of Yang and Lin[1].

A New Approach for Modeling Electro-Osmotic Flow Micro-Cooling Systems

The problem of entropy generation due to heat transfer in an electro-osmotic flow with circular type area section micro-channels for small values of electro-kinetic parameterκα(whereκis the Debye length andais the radius of the micro-channel) is investigated analytically. The momentum and energy conservation equations in cylindrical coordinates for an electro-osmotic flow are derived in non-dimensional form. The momentum equation is solved and velocity distribution in terms of modified Bessel function of the first kind is obtained. An approximation is used for the Bessel function of first kind. Considering the approximate velocity profile, the energy equation including viscous dissipation effects is solved to obtain the temperature distribution in terms of parametric values of the electro-kinetic parameter and the Brinkman number. A uniform surface heat flux boundary condition is considered. The importance of this investigation is development of an engineering simple method of design for electro-osmotic circular micro-electronic cooling systems and possible optimizations of these kinds of flows with respect to the second-law of thermodynamics through the micro-coolers.

Korelasi Empiris Perpindahan Kalor Dua Fase (Air Udara) Aliran Kantung Gas dalam Pipa Horisontal Sirkular

This research aimed at determining the heat transfer correlation of a heated non-boiling two phase flow consisting of air and water in a horizontal round pipe. The test section was a 24 mm ID cooper pipe with a length 1 m, and the heat transfer data a uniform wall heat flux boundary condition ranging from about 5968,17 to 10503,97 W/m2. From this research, it can be concluded that heat transfer coefficient of two-phase flow of bubble flow is increase as increase of gas superficial velocity and liquid superfisial velocity. The comparison deviation standart between heat transfer coefficient of two-phase flow experimental and heat transfer coefficient of two-phase flow correlation that proposed is 34%, but this deviation is still in limited value for experimental research.

A way to explain the thermal boundary effects on laminar convection through a square duct

A way using the reformulation of the energy conservation equation in terms of heat flux to explain the thermal boundary effects on laminar convective heat transfer through a square duct is presented. For a laminar convection through a square duct, it explains that on the wall surface, the velocity is zero, but convection occurs for uniform wall heat flux (UWHF) boundary in the developing region due to the velocity gradient term; for uniform wall temperature (UWT) boundary, only diffusion process occurs on the wall surface because both velocity and velocity gradient do not contribute to convection; for UWHF, the largest term of the gradient of velocity components (the main flow velocity) on the wall surface takes a role in the convection of the heat flux normal to the wall surface, and this role exists in the fully developed region. Therefore, a stronger convection process occurs for UWHF than for UWT on the wall surface. The thermal boundary effects on the laminar convection inside the flow are also detailed.

Numerical Modeling of Pressure-Driven Nitrogen Slip Flow in Long Rectangular Microchannels

This article addresses some analytical and numerical modeling issues regarding the simulation of pressure-driven nitrogen slip flow in long microchannels. The main motivation for the study is to overcome the intense computational effort required by the large computational domain and the slow downstream variation and convergence in this problem, and to address some of the existing concerns with current models. A parallel solver is developed and used along with a serial version to obtain the steady state solution. This approach is found to provide an efficient and accurate solution to the problem. A comparison with earlier results is used for validation, as well as for justifying this hybrid approach. Some implemental issues related to the parallel algorithm are discussed and solved. The effects of variable properties, rarefaction, and the source terms in energy equation are determined and are found to be significant, particularly for the case of uniform wall heat flux boundary condition.

Micro-scale thermo-fluidic transport in two immiscible liquid layers subject to combined electroosmotic and pressure-driven transport

A theoretical analysis is presented for fully developed convective heat transfer in two immiscible fluid layers confined within parallel plate microchannels subject to combined effects of axial pressure gradients and imposed electrical fields. Assuming desperate zeta potentials at the interfaces thus formed, closed-form expressions are derived for the velocity and temperature distributions under fully developed conditions, with uniform wall heat flux boundary conditions. For the heat transfer analysis, the viscous dissipation effects are neglected as compared to the Joule heating effects. Results are subsequently obtained for different ranges of the ratios of various electrical properties of the two fluid layers and various relative strengths of the ratios of the electrical fields and the imposed pressure gradients. These results demonstrate the effects of the applied electric fields and pressure gradients, presence of external heat source or sink and interfacial positions on the temperature distributions in the two layers and the corresponding Nusselt numbers.

Contribution Analysis of Dimensionless Variables for Laminar and Turbulent Flow Convection Heat Transfer in a Horizontal Tube Using Artificial Neural Network

The artificial neural network (ANN) method has shown its superior predictive power compared to the conventional approaches in many studies. However, it has always been treated as a “black box” because it provides little explanation on the relative influence of the independent variables in the prediction process. In this study, the ANN method was used to develop empirical correlations for laminar and turbulent heat transfer in a horizontal tube under the uniform wall heat flux boundary condition and three inlet configurations (re-entrant, square-edged, and bell-mouth). The contribution analysis for the dimensionless variables was conducted using the index of contribution defined in this study. The relative importance of the independent variables appearing in the correlations was examined using the index of contribution based on the coefficient matrices of the ANN correlations. For the turbulent heat transfer data, the Reynolds and Prandtl numbers were observed as the most important parameters, and the length-to-diameter and bulk-to-wall viscosity ratios were found to be the least important parameters. The method was extended to analyze the more complicated forced and mixed convection data in developing laminar flow. The dimensionless parameters influencing the heat transfer in this region were the Rayleigh number and the Graetz number. The contribution analysis clearly showed that the Rayleigh number has a significant influence on the mixed convection heat transfer data, and the forced convection heat transfer data is more influenced by the Graetz number. The results of this study clearly indicated that the contribution analysis method can be used to provide correct physical insight into the influence of different variables or a combination of them on complicated heat transfer problems.

Transitional Heat Transfer in Plain Horizontal Tubes

In this study, the heat transfer behavior in the transition region for plain horizontal tubes under a uniform wall heat flux boundary condition is discussed in detail. In particular, the influence of inlet configuration and free convection superimposed on the forced convection (or mixed convection) at the start and end of the transition region and the magnitude of heat transfer are addressed. The available correlations to predict the heat transfer coefficient in the transition region are reviewed, and their performance are evaluated based on 1290 experimental data points obtained under a wide range of experimental conditions. Appropriate correlations for the mixed and forced convection transition regions are recommended. Finally, a flow regime map for determination of the boundary between forced and mixed convection in horizontal tubes with different inlets is presented.

Fully developed laminar flow and heat transfer in smooth trapezoidal microchannel

Abstract Numerical analysis of fully developed laminar slip flow and heat transfer in trapezoidal micro-channels has been studied with uniform wall heat flux boundary conditions. Through coordinate transformation, the governing equations are transformed from physical plane to computational domain, and the resulting equations are solved by a finite-difference scheme. The influences of velocity slip and temperature jump on friction coefficient and Nusselt number are investigated in detail. The calculation also shows that the aspect ratio and base angle have significant effect on flow and heat transfer in trapezoidal micro-channel.

EFFECT OF CHANNEL CONFINEMENT ON THE TWO-DIMENSIONAL LAMINAR FLOW AND HEAT TRANSFER ACROSS A SQUARE CYLINDER

ABSTRACT The flow structure and heat transfer characteristics of a square cylinder in cross flow are investigated numerically for both unconfined and channel-confined (blockage ratios, 10–50% in steps of 10%) flow situations at Reynolds numbers of 50, 100, and 150, and a Prandtl number of 0.7. As the blockage ratio is increased, the Reynolds number for the onset of vortex shedding increases and then decreases. Increases in Strouhal number, drag coefficient, pumping power, and cylinder Nusselt number are also observed with increasing blockage ratio. The enhancement of cylinder Nusselt number due to channel confinement is investigated for uniform heat flux and constant cylinder temperature boundary conditions.

HEAT TRANSFER CHARACTERISTICS OF GASEOUS FLOW IN LONG MINI- AND MICROTUBES

A numerical procedure which solves the compressible boundary-layer equations is used to study the heat transfer characteristics of gaseous laminar and turbulent flows in mini- and microtubes. Uniform wall heat flux and isothermal wall boundary conditions are simulated. The numerical results show that when the tube diameter is large and the Reynolds number is small, the numerically predicted Nusselt numbers agree with conventional correlations. As the tube diameter gets small and the Reynolds number gets large, the heat transfer characteristics become quite different from those given by the conventional correlations. The reasons for the differences can be explained satisfactorily by the compressibility and the accompanying strong expansion process of the gas along the longitudinal axis. The parabolic character of the boundary-layer equations renders the numerical procedure a very efficient, accurate, and robust tool for studying compressible microtube flows. It took less than 3 min for this numerical procedure to run a typical case on a Pentium IV 2.4 G PC and is at least two to three orders of magnitude faster than the full Navier-Stokes simulations.