A numerical study of the unsteady flow and heat transfer in a transitional confined slot jet impinging on an isothermal surface

A numerical finite-difference approach was used to compute the steady and unsteady flow and heat transfer due to a confined two-dimensional slot jet impinging on an isothermal plate. The jet Reynolds number was varied from Re=250 to 750 for a Prandtl number of 0.7 and a fixed jet-to-plate spacing of H/W=5. The flow was found to become unsteady at a Reynolds number between 585 and 610. In the steady regime, the stagnation Nusselt number increased monotonically with Reynolds number, and the distribution of heat transfer in the wall jet region was influenced by flow separation caused by re-entrainment of the spent flow back into the jet. At a supercritical Reynolds number of 750 the flow was unsteady and the net effect in the time mean was that the area-averaged heat transfer coefficient was higher compared to what it would have been in the absence of jet unsteady effects. The unsteady jet exhibited a dominant frequency that corresponded to the formation of shear layer vortices at the jet exit. Asymmetry in the formation of the vortex sheets caused deformation or buckling of the jet that induced a low-frequency lateral jet “flapping” instability. The heat transfer responds to both effects and leads to a broadening of the cooled area.     

A numerical investigation on the fluid flow and heat transfer in the confined impinging slot jet in the low Reynolds number region for different channel heights

The numerical solution is obtained for unsteady two-dimensional fluid flow and heat transfer in a confined impinging slot jet using the finite volume method. In order to consider the effect of Reynolds number and height ratio on the flow and temperature fields in the channel, the numerical simulations were performed for different Reynolds numbers of 50–500 and different height ratios of 2–5. The critical Reynolds number, beyond which the flow and thermal fields change their state from steady to unsteady, depends on the Reynolds number and height ratio. The unsteadiness gives a big impact on the flow and temperature fields and as a result the pressure coefficient, skin friction coefficient and Nusselt number in the unsteady region show different characteristics from those in the steady region.     

A numerical study of fluid flow and heat transfer in eccentric curved annuli

In this article fluid flow and heat transfer in curved eccentric annuli are studied numerically. A second order finite difference method based on the Projection algorithm is implemented to solve the governing equations including the full Navier–Stokes, the continuity, and the energy equations in a toroidal coordinate system. For convenience a bipolar based toroidal coordinate system is employed to discretize the governing equations in the annulus domain using a uniform staggered grid which is required in finite difference methods. Considering hydrodynamically and thermally fully developed conditions, the effects of different physical parameters such as eccentricity, Dean number, curvature, Prandtl number on the flow field and thermal characteristics at different thermal boundary conditions are investigated in detail. It is also shown that in contrast to straight eccentric annuli, heat transfer rates can be augmented in the eccentric curved annuli comparing with the straight concentric annuli at the large dean numbers.     

Experimental and numerical investigation of natural convection effects on confined impinging jet heat transfer

The influence of natural convection on the local and average heat transfer at increasing temperature differences between the jet and the target plate from confined impinging jets has been experimentally and numerically investigated. Local Nusselt numbers were obtained numerically for jet Reynolds numbers in the range of 250–1000, and jet to target spacings of 2, 4, 8, 12 jet diameters at various modified Grashof numbers, to determine the effect of buoyancy induced natural convection. To determine the overall effect of natural convection on the average heat transfer, experiments have been conducted for Re numbers in the range 250–5000 and dimensionless jet to target spacing 2, 4, 6, 8, and 12 at increasing modified Grashof numbers. It has been determined that buoyancy induced natural convection might have opposing or assisting influence on local heat transfer at different locations of the target plate. It has also been shown that especially at low jet inlet velocities the average heat transfer coefficient at the highest modified Grashof number, where the natural convection is effective, is higher than the value corresponding to the lowest Grashof number at which buoyancy effects are negligible, by as much as 37%.     

Analytical and numerical solutions of heat and mass transfer of boundary layer flow in the presence of a transverse magnetic field

The present investigation aims to study the effect of a transverse magnetic field with the presence of an adverse pressure gradient on the two‐dimensional laminar incompressible boundary layer flow over a flat plate. Using appropriated similarity transformations, the partial differential equations governing the studied problems are transformed into the ordinary nonlinear differential equations. Thereafter, these equations are solved numerically and analytically using the fourth‐order Runge‐Kutta method featuring shooting technique and the Adomian decomposition method, respectively. Obtained results reveal an excellent agreement between analytical and numerical data for temperature and concentration profiles.     

Application of radially varying magnetic field on a peristaltic flow of non‐Newtonian fluid in the presence of heat and mass transfer

This article deals with the variable MHD effects on the peristaltic flow of a non‐Newtonian fluid in the presence of heat and mass transfer. The walls of annulus are maintained at different temperatures. Continuity, momentum concentration, and energy equations are utilized in the mathematical analysis. Two types of solutions, namely, the exact and numerical, are derived. These solutions are compared and discussed. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20314     

Thermal flow of fluid with magnetic particles in the presence of magnetic field

A magnetic fluid is a stable colloidal solution composed of magnetic particles, surfactant, and a carrier fluid. Magnetic fluids have numerous applications. In this study, we used the computational fluid dynamics model to simulate the behavior of a magnetic fluid in a 2-D square under different conditions such as different positions and intensities of the magnetic source. The preliminary model was established and used in conjunction with experimental data obtained from a present study, in order to determine the influence of particle size and mass fraction on fluid behavior. Our results show that particles with a smaller size have better ability to dissipate heat, and a larger mass fraction would provide a stronger driving force which leads to the velocity and temperature profile. We anticipate that our model would be useful to develop newer applications of magnetic fluids and magnetic flow.     

Experimental and numerical study of convective heat transfer and fluid flow in twisted oval tubes

Twisted oval tube heat exchanger is a type of heat exchanger aims at decreasing the pressure drop of the shell side. In the present study, heat transfer and pressure drop performances of twisted oval tube have been studied experimentally and numerically. The experimental study of the twisted oval tube shows that heat transfer process can be enhanced but also with an increasing of pressure drop when compared with the smooth round tube. The effects of geometrical parameters on the performance of the twisted oval tube have been analyzed numerically. The result reveals that the heat transfer coefficient and friction factor both increase with the increasing of axis ratio a/b, while both decrease with the increasing of twist pitch length P. The influence of a/b and P on the overall performance of the twisted oval tubes are also studied. Aiming at obtaining the heat transfer enhancement mechanism of the twisted oval tube, secondary flow, total velocity and temperature distributions of flow section are given. From the analysis it can be concluded that the emergence of twist in the twisted oval tube results in secondary flow. It exists in the form of spiral flow when a/b is big, but in the form of up and down when a/b is small. It is this secondary flow that changes the total velocity and temperature distributions of the twisted oval tube when compared with a smooth oval tube with the same sectional geometric parameters. Then the synergy angle between velocity vector and temperature gradient is reduced and the heat transfer process is enhanced.     

受限空气射流冲击矩形柱鳍热沉换热实验研究

采用实验方法研究了受限空气冲击射流与矩形柱鳍热沉相结合的散热方式应用于芯片冷却的换热规律,采用最小二乘法对实验数据进行了拟合,并最终获得平均努塞尔数关于雷诺数、喷口高度-孔径比及普朗特数的实验准则方程。在此基础上将这种散热方式与其他空冷方式进行了换热能力的比较,结果表明此种散热方式的换热能力大大超过其他空冷方式。最后,对实验系统误差进行了分析,根据误差传递理论求得的平均努塞尔数的实验相对误差不超过6%。     

Effects of geometric parameters on confined impinging jet heat transfer

The objective of this work is to carry out a numerical investigation to examine the effects of geometric parameters on the confined impinging jet heat transfer. Parameters such as Nusselt number, Reynolds number, H/W have been studied. Nozzle width H ranges from 0.6 mm to 2 mm, and nozzle-to-plate spacing W ranges from 0.5 mm to 10 mm. The jet flow is in the range of laminar flow with Reynolds number from 26.8 to 1000. This paper presents distributions of target surface temperature, local and average Nusselt number on the target plate. Pressure drop for different H/W is also obtained. This study can provide useful information to the application of impinging jet heat transfer in industry.     

Three-dimensional modelling of fluid flow and heat transfer in micro-channels with synthetic jet

A three-dimensional computational model was developed to investigate the effect of synthetic jet interaction with cross flow in micro-channel on the cooling of microchip. A range of parametric studies by varying heat fluxes at the surface of the top of the silicon wafer and membrane oscillating amplitudes was conducted. The resulting complex, conjugate heat transfer through the silicon substrate was analysed. When the actuator was switched on, noticeable temperature drop was observed at all points in the substrate. Quasi steady states have been reached for the presented results which indicated the available cooling potential of single synthetic jet actuator.     

Numerical study of ferrofluid flow and heat transfer in the presence of a non‐uniform magnetic field in rectangular microchannels

This paper presents a numerical investigation of the hydro‐thermal behavior of a ferrofluid in rectangular minichannels in the presence of a non‐uniform magnetic field using a two‐phase mixture model and control volume technique. Effects of increasing the diameter of nanoparticles, and channel aspect ratio have also been studied. It is concluded that the magnetic field with a negative gradient increases the Nusselt number and the rate of this increment depends on the channel aspect ratio. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21004     

Unsteady flow of viscous incompressible fluid with temperature-dependent viscosity due to a rotating disc in presence of transverse magnetic field and heat transfer

This paper studies the effect of a magnetic field and temperature-dependent viscosity on the unsteady flow and heat transfer for a viscous laminar incompressible and electrically conducting fluid due to an impulsively started rotating infinite disc. The unsteady axisymmetric boundary layer equations are solved using three methods, namely, (i) perturbation solution for small time, (ii) asymptotic analysis for large time and (iii) finite difference method together with Keller box elimination technique for intermediate times. The solutions are obtained in terms of local radial skin friction, local tangential skin friction, and local rate of heat transfer at the surface of the disc, for different values of the pertinent parameters: the Prandtl number Pr, the viscosity variation parameter ε and magnetic field parameter m. The computed dimensionless velocity and temperature profiles for Pr=0.72 are shown graphically for different values of ε and m.     

Simultaneous integration,control and enhancement of both fluid flow and heat transfer in small scale heat exchangers: A numerical study

Compactness, efficiency and control of heat exchangers are of great interest in many processes. A technological breakthrough must be achieved to go further in their ability to respond to needs. A new concept of heat exchanger is proposed. It consists in dynamically deforming at least one of the walls of a low hydraulic diameter channel. Heat transfer and mass flow rate enhancements are investigated in single-phase flow. When the deformation is a progressive wave with a relative amplitude of 98.5% and frequency of 50 Hz, it generates a flow having a mass velocity of up to 510 kg·m^− 2·s^− 1. Although the Reynolds number is low the heat transfer coefficient is enhanced by up to 450% compared to a straight channel.     

Effect of magnetic obstacle on fluid flow and heat transfer in a rectangular duct

The vortex dynamics behind various magnetic obstacles and characteristics of heat transfer are investigated using a three-dimensional model. In the numerical study, the magnet width (M_y) is alterable to investigate the instability, Strouhal number, wake structure behind various magnetic obstacles and percentage increment of the overall heat transfer for a wide range of constrainment factors (0.08 ≤ κ ≤ 0.26), Reynolds numbers (400 ≤ Re ≤ 900) and interaction parameters (9 ≤ N ≤ 15). For all constrainment factors, the fundamental frequency (f) is uniform for a particular value of Reynolds number. Downstream cross-stream mixing due to vortex shedding enhances the wall-heat transfer and the maximum value of percentage increment of the overall heat transfer (HI) is about 20.2%. However, the pressure drop penalty (ΔP_penalty) is not increasingly dependent on interaction parameter when Re and κ remain constant.     

A numerical study of single-phase convective heat transfer in microtubes for slip flow

The steady-state convective heat transfer for laminar, two-dimensional, incompressible rarefied gas flow in the thermal entrance region of a tube under constant wall temperature, constant wall heat flux, and linear variation of wall temperature boundary conditions are investigated by the finite-volume finite difference scheme with slip flow and temperature jump conditions. Viscous heating is also included, and the solutions are compared with theoretical results where viscous heating has been neglected. For these three boundary conditions for a given Brinkman number, viscous effects are presented in the thermal entrance region along the channel. The effects of Knudsen and Brinkman numbers on Nusselt number are presented in graphical and tabular forms in the thermal entrance region and under fully developed conditions.     

Studying the effect of the force convection boundary condition and radius magnetic field on fluid flow and heat transfer between coaxial pipes

An investigation of the heat transfer of Newtonian fluid flow through coaxial two pipes with variable radius ratio has been conducted with the boundary conditions of forced convection on the inner pipe walls and a radius magnetic field. This paper presents an exact analytical solution to the momentum equation and a novel semi-analytic collocation method for solving the full-term energy equation that takes Joule heating into account as well as viscous dissipation. Based on the results of the numerical fourth-order Runge–Kutta method, it was found that increasing the magnetic parameter decreased the amount of friction on the surface of the pipe walls and the rate of heat transfer. As the radius ratio of the two pipes increases, so does the skin friction and heat transfer rate on the internal pipe walls. As Eckert (Ec) and Prandtl (Pr) numbers increase, the mean temperature as well as the dimensionless temperature between the two pipes increases. The increase in Biot number (Bi) has the opposite impact on the mean temperature. As Ec, Pr, and Bi increase, so does the rate of heat transfer on the inner wall of the pipe.     

Experimental study on heat transfer augmentation in a double pipe heat exchanger utilizing jet vortex flow

This paper examines experimentally the effect of jet vortex technology on enhancing the heat transfer rate within a double pipe heat exchanger by supplying the heat exchanger with water at different vortex strengths. A vortex generator with special inclined holes with different inlet angles was designed, manufactured, and integrated within the heat exchanger. In this study, four levels of Reynolds number for hot water in the annulus (Re_h) were used, namely, 10,000; 14,500; 18,030; and 19,600. Similarly, four levels of Reynolds number for cold water in the inner tube (Re_c) were used, namely, 12,000; 17,500; 22,500; and 29,000. As for the inlet flow angle (θ), four different levels were selected, namely, 0°, 30°, 45°, and 60°. The temperature along the heat exchanger was measured utilizing 34 thermocouples installed along the heat exchanger. It was found that increasing the inlet flow angle (θ) and/or the Reynolds number results in an increase in the local Nusselt number, the overall heat transfer coefficient, and the ratio of friction factor. It is revealed that the percentage increase in the average Nusselt number due to swirl flow compared to axial flow was 10%, 40%, and 82% for an inlet flow angle of 30°, 45°, and 60°, respectively.     

Numerical study of fluid flow and heat transfer in microchannel cooling passages

Numerical investigation was conducted for fluid flow and heat transfer in microchannel cooling passages. Effects of viscosity and thermal conductivity variations on characteristics of fluid flow and heat transfer were taken into account in theoretical modeling. Two-dimensional simulation was performed for low Reynolds number flow of liquid water in a 100 μm single channel subjected to localized heat flux boundary conditions. The velocity field was highly coupled with temperature distribution and distorted through the variations of viscosity and thermal conductivity. The induced cross-flow velocity had a marked contribution to the convection. The heat transfer enhancement due to viscosity-variation was pronounced, though the axial conduction introduced by thermal-conductivity-variation was insignificant unless for the cases with very low Reynolds numbers.