Comparative evaluation of three heat transfer enhancement strategies in a grooved channel

 Results of a comparative evaluation of three heat transfer enhancement strategies for forced convection cooling of a parallel plate channel populated with heated blocks, representing electronic components mounted on printed circuit boards, are reported. Heat transfer in the reference geometry, the asymmetrically heated parallel plate channel, is compared with that for the basic grooved channel, and the same geometry enhanced by cylinders and vanes placed above the downstream edge of each heated block. In addition to conventional heat transfer and pressure drop measurements, holographic interferometry combined with high-speed cinematography was used to visualize the unsteady temperature fields in the self-sustained oscillatory flow. The locations of increased heat transfer within one channel periodicity depend on the enhancement technique applied, and were identified by analyzing the unsteady temperature distributions visualized by holographic interferometry. This approach allowed gaining insight into the mechanisms responsible for heat transfer enhancement. Experiments were conducted at moderate flow velocities in the laminar, transitional and turbulent flow regimes. Reynolds numbers were varied in the range Re = 200–6500, corresponding to flow velocities from 0.076 to 2.36 m/s. Flow oscillations were first observed between Re = 1050 and 1320 for the basic grooved channel, and around Re = 350 and 450 for the grooved channels equipped with cylinders and vanes, respectively. At Reynolds numbers above the onset of oscillations and in the transitional flow regime, heat transfer rates in the investigated grooved channels exceeded the performance of the reference geometry, the asymmetrically heated parallel plate channel. Heat transfer in the grooved channels enhanced with cylinders and vanes showed an increase by a factor of 1.2–1.8 and 1.5–3.5, respectively, when compared to data obtained for the basic grooved channel; however, the accompanying pressure drop penalties also increased significantly. Received on 5 April 2001     

MHD Couette two-fluid flow and heat transfer in presence of uniform inclined magnetic field

The MHD Couette flow of two immiscible fluids in a parallel plate channel in the presence of an applied electric and inclined magnetic field is investigated in the paper. One of the fluids is assumed to be electrically conducting, while the other fluid and the channel plates are assumed to be electrically insulating. Separate solutions with appropriate boundary conditions for each fluid are obtained and these solutions are matched at the interface using suitable matching conditions. The partial differential equations governing the flow and heat transfer are transformed to ordinary differential equations and closed-form solutions are obtained in both fluid regions of the channel. The results for various values of the Hartmann number, the angle of magnetic field inclination, the loading parameter and the ratio of the heights of the fluids are presented graphically to show their effect on the flow and heat transfer characteristics.     

Experimental study and artificial neural network modeling of unsteady laminar forced convection in a rectangular duct

In this study, an artificial neural network (ANNs) for the prediction of unsteady heat transfer in a rectangular duct was studied. The ANNs has been applied for the unsteady heat transfer in a rectangular duct. An experimental study has been carried out to investigate the axial variation of inlet temperature and the impact of inlet frequency on decay indices in the thermal entrance region of a parallel plate channel. The investigation was conducted with laminar forced flows for Reynolds numbers ranging from 1,120 to 2,200 while the inlet heat input frequency varied from 0.02 to 0.24 Hz. The results revealed that the ANNs can be used for modeling unsteady heat transfer in the duct. The accuracy between experimental and ANNs approach results was achieved with a mean absolute relative error less than 39%.     

Natural convection effects on impulsively started inclined plate with heat and mass transfer

The present investigation, involving the simultaneous heat and mass transfer is concerned with a numerical study of transient natural convection flow past an impulsively started inclined plate. Crank-Nicolson implicit finite difference method is used to solve the unsteady, non-linear and coupled governing equations. In order to check the accuracy of the numerical results, the present study is compared with available exact solution and are found to be in good agreement. Numerical results are obtained for various parameters. The steady-state velocity, temperature and concentration profiles, local and average skin friction, local and average Nusselt number, local and average Sherwood number are shown graphically. It is observed that local wall shear stress decreases as an angle of inclination { decreases.     

Vortex mechanism of heat transfer enhancement in a channel with spherical and oval dimples

Vortex mechanism of heat transfer enhancement in a narrow channel with dimples has been investigated numerically using LES and URANS methods. The flow separation results in a formation of vortex structures which significantly enhance heat transfer on dimpled surfaces leading to a small increase in pressure loss. The heat transfer can be significantly increased by rounding the dimple edge and use of oval dimples. To get a deep insight into flow physics LES is performed for single phase flow in a channel with a spherical dimple. The instantaneous vortex formation and separation are investigated in and around the dimple area. Considered are Reynolds numbers (based on dimple print diameter) Re_D = 20,000 and Re_D = 40,000 the depth to print diameter ratio of Δ = 0.26. Frequency analysis of LES data revealed the presence of dominating frequencies in unsteady flow oscillations. Direct analysis of the flow field revealed the presence of coherent vortex structure inclined to the mean flow. The structure changes its orientation in time causing the long period oscillations with opposite-of-phase motion. Three dimensional proper orthogonal decomposition (POD) analysis is carried out on LES pressure and velocity fields to identify spatio-temporal structures hidden in the random fluctuations. Tornado-like spatial POD structures have been determined inside dimples.     

Unsteady natural convection Couette flow of heat generating/absorbing fluid between vertical parallel plates filled with porous material

The extended Brinkman Darcy model for momentum equations and an energy equation is used to calculate the unsteady natural convection Couette flow of a viscous incompressible heat generating/absorbing fluid in a vertical channel(formed by two infinite vertical and parallel plates) filled with the fluid-saturated porous medium.The flow is triggered by the asymmetric heating and the accelerated motion of one of the bounding plates.The governing equations are simplified by the reasonable dimensionless parameters and solved analytically by the Laplace transform techniques to obtain the closed form solutions of the velocity and temperature profiles.Then,the skin friction and the rate of heat transfer are consequently derived.It is noticed that,at different sections within the vertical channel,the fluid flow and the temperature profiles increase with time,which are both higher near the moving plate.In particular,increasing the gap between the plates increases the velocity and the temperature of the fluid,however,reduces the skin friction and the rate of heat transfer.     

Adaptive finite element method for high-speed flow-structure interaction

An adaptive finite element method for high-speed flow-structure interaction is presented. The cell-centered finite element method is combined with an adaptive meshing technique to solve the Navier-Stokes equations for high-speed compressible flow behavior. The energy equation and the quasi-static structural equations for aerodynamically heated structures are solved by applying the Galerkin finite element method. The finite element formulation and computational procedure are described. Interactions between the high-speed flow, structural heat transfer, and deformation are studied by two applications of Mach 10 flow over an inclined plate, and Mach 4 flow in a channel. The project supported by the Thailand Research Fund (TRF)     

Transient mixed convection flow of a second grade viscoelastic fluid past an inclined backward facing step

The transient mixed convection of a second-grade viscoelastic fluid past an inclined backward facing step was studied numerically. The combined effects of the Reynolds number, the elastic effect, the inclined angle of the flow channel on the reattachment length, and the phenomena of heat transfer are examined during the development of the flow field. The Gauss-Seidal method with successive over relaxation was implemented to solve the stream-vorticity and energy equations. The results indicate that the reattachment length increases to the maximum as the inclined angle increases up to 150° or 180°. At these cases, the point of reattachment is close to the point of the local maximum value of Nu_x or is overshooting it. It is observed that the reattachment length increases as the Reynolds number increases or the elastic coefficient decreases. In the meantime, the contact point of the isotherm on the upper plate moves upward and is close to upstream flow as the inclined angle is around 150°.     

Investigation of temperature statistic characteristics in turbulent water flow with unsteady heat transfer

The results of an experimental study of a temperature field and its statistical characteristics in turbulent water flow upon a sudden change of heat generation in the channel wall are reported. Measurements were performed in 5 mm × 40 mm, 10 mm × 40 mm, and 20 mm × 40 mm channels in the regions of thermal stabilization and stabilized heat transfer at Reynolds numbers of (0.8–6.8) × 10⁴. The measurement results are generalized using a dimensionless time scale. The results of the calculation of heat transfer coefficients at unsteady heat transfer are presented.     

Momentum and heat transfer of a special case of the unsteady stagnation-point flow

This paper investigates the unsteady stagnation-point flow and heat transfer over a moving plate with mass transfer,which is also an exact solution to the unsteady Navier-Stokes(NS)equations.The boundary layer energy equation is solved with the closed form solutions for prescribed wall temperature and prescribed wall heat flux conditions.The wall temperature and heat flux have power dependence on both time and spatial distance.The solution domain,the velocity distribution,the flow field,and the temperature distribution in the fluids are studied for different controlling parameters.These parameters include the Prandtl number,the mass transfer parameter at the wall,the wall moving parameter,the time power index,and the spatial power index.It is found that two solution branches exist for certain combinations of the controlling parameters for the flow and heat transfer problems.The heat transfer solutions are given by the confluent hypergeometric function of the first kind,which can be simplified into the incomplete gamma functions for special conditions.The wall heat flux and temperature profiles show very complicated variation behaviors.The wall heat flux can have multiple poles under certain given controlling parameters,and the temperature can have significant oscillations with overshoot and negative values in the boundary layers.The relationship between the number of poles in the wall heat flux and the number of zero-crossing points is identified.The difference in the results of the prescribed wall temperature case and the prescribed wall heat flux case is analyzed.Results given in this paper provide a rare closed form analytical solution to the entire unsteady NS equations,which can be used as a benchmark problem for numerical code validation.     

Effects of an upstream inclined rod on the circular cylinder–flat plate junction flow

An inclined rod is installed upstream of a circular cylinder mounted on a flat plate to mitigate the horseshoe vortices in the junction flow. Smoke-wire visualization, hot-wire velocity measurement and surface pressure measurement are employed to study the effects of the inclined rod on the laminar and turbulent junction flows. The results show a properly placed inclined rod can significantly weaken the horseshoe vortices in front of the cylinder, depress the unsteady oscillation of the vortex system, change the separation position on the flat plate and narrow the wake of the cylinder. The inclined rod method provides a promising way to suppress the horseshoe vortices in the junction flow because of its effectiveness and its easiness to implement and adjust to fit different flow conditions.     

Conjugate conduction-natural convection in an enclosure with time-periodic sidewall temperature and inclination

The unsteady conjugate conduction-natural convection in enclosure is of great theoretical significance and is widely encountered in engineering applications in the areas of fluid dynamics and heat transfer. However, there are relatively few efforts to investigate the unsteady flow physics and heat transfer characteristics in the inclined enclosure of finite thickness walls. In the present work, this problem is numerically investigated by a high accuracy multidomain temporal-spatial pseudospectral method. The enclosure is filled with Boussinesq fluid and is bounded by four finite thickness and conductive walls; one of the vertical sidewall is exposed to time-periodic temperature environment while the opposite sidewall holds constant temperature; the top and bottom walls are assumed to be adiabatic. Particular efforts are focused on the effects of three types of influential factors: the wall thermophysical properties, the time-periodic temperature patterns and the inclination, and the time-periodic flow patterns and heat transfer characteristics are presented. Numerical results reveal that within the present parameter range, the heat transfer rate increases almost linearly with the thermal conductivity ratio and thermal diffusivity ratio but decreases with the inclination angle. Moreover, the heat transfer could be enhanced or weakened by selecting different temperature pulsating period in the case of finite thickness wall, while it is always enhanced if the walls are zero thickness. The back heat transfer and heat transfer resonance phenomena are observed, and their relationships with the time-periodic flow patterns and temperature distributions are analyzed. The findings are helpful to the understandings of the fluid flow and heat transfer mechanisms in the related enclosure configurations, and may be of engineering use in thermal design improvement.     

Unsteady Free Convection along an Infinite Vertical Flat Plate Embedded in a Stably Stratified Fluid-Saturated Porous Medium

The problem of unsteady free convection heat transfer from a one-dimensional (parallel) flow along an infinite vertical flat plate embedded in a thermally stratified fluid-saturated porous medium is considered. Flows are induced by a sudden change in the arbitrary temporal plate temperature. By a formal reduction of the corresponding boundary value problems to well-known Fourier heat conduction problems, analytical solutions of the Darcy and energy equations are obtained. Several special cases are discussed in detail.     

Flow visualisation in inclined louvered fins

In this study the flow within an interrupted fin design, the inclined louvered fin, is investigated experimentally through visualisation. The inclined louvered fin is a hybrid of the offset strip fin and standard louvered fin, aimed at improved performance at low Reynolds numbers for compact heat exchangers. The flow behaviour is studied in six geometrically different configurations over a range of Reynolds numbers and quantified using the concept of ‘fin angle alignment factor’. The transition from steady laminar to unsteady flow was studied in detail. The fin geometry had a very large impact on the transitional flow behaviour, especially on vortex shedding.     

Theoretical investigation on thermal performance of heat pipe flat plate solar collector with cross flow heat exchanger

Based on the heat transfer characteristics of absorber plate and the heat transfer effectiveness-number of heat transfer unit method of heat exchanger, a new theoretical method of analyzing the thermal performance of heat pipe flat plate solar collector with cross flow heat exchanger has been put forward and validated by comparisons with the experimental and numerical results in pre-existing literature. The proposed theoretical method can be used to analyze and discuss the influence of relevant parameters on the thermal performance of heat pipe flat plate solar collector.     

Turbulent flow structure and heat transfer in an inclined bubbly flow. Experimental and numerical investigation

The effect of channel inclination on the variation in the wall shear stress and the heat transfer in a two-phase bubbly flow in a rectangular channel is experimentally and numerically investigated. The wall friction was measured using the electrodiffusion method and the temperature was measured by tiny platinum resistance thermometers. The model is based on the system of RANS equations with account for the back influence of the bubbles on the flow characteristics. Flow turbulence is calculated according to the model of transport of the Reynolds stress tensor components. It is shown that in the gas-liquid flow the angle of the channel inclination to the horizon can have a considerable effect on the friction and the heat transfer. The greatest friction and heat transfer values correspond to the angles of channel inclination ranging from 30 to 50°. In the inclined two-phase bubbly flow the shear stress enhancement on the wall amounts to 30% and that of the heat transfer to 15%. A friction and heat transfer reduction to 10 and 25%, respectively, is noticed in near-horizontal flows.     

Mixed convective heat transfer characteristics and mechanisms in structured packed beds

The structured packed bed is considered a promising reactor owing to its low pressure drop and good heat transfer performance. In the heat transfer process of thermal storage in packed beds, natural convection plays an important role. To obtain the mixed convective heat transfer characteristics and mechanisms in packed beds, numerical simulations and coupling analyses were carried out in this study on the unsteady process of fluid flow and heat transfer. A three-dimensional model of the flow channel in the packed bed was established, and the Navier–Stokes equations and Laminar model were adopted for the computations. The effects of the driving force on fluid flow around a particle were studied in detail. The differences in velocity and density distributions under different flow directions due to effect of the aiding flow or opposing flow were intuitively demonstrated and quantitatively analyzed. It was found that the driving force strengthens the fluid flow near the particle surface when aiding flow occurs and inhibits the fluid flow when opposing flow occurs. The boundary layer structure was changed by the natural convection, which in turn influences the field synergy angle. For the aiding flow, the coordination between the velocity and density fields is higher than that for the opposing flow. By analysis the effects of physical parameters on mixed convective heat transfer, it is indicated that with an increase in the fluid-solid temperature difference or the particle diameter, or a decrease in the fluid temperature, the strengthening or inhibiting effect of natural convection on the heat transfer became more significant.     

Numerical study of steady/unsteady flow and heat transfer in porous media using a characteristics-based matrix-free implicit FV method on unstructured grids

In this study, a high-resolution characteristic-based finite-volume (FV) method on unstructured grids [Int. J. Numer. Method Eng. 50 (2001) 11; Int. J. Heat Fluid Flow 21 (2000) 432] is extended by a matrix-free implicit dual-time stepping scheme for the numerical simulation of steady and unsteady flow and heat transfer with porous media. The method has been used to study the characteristics of a complex problem: flow and heat transfer in a channel with multiple discrete porous blocks, which was originally proposed by Huang and Vafai [J. Thermophys. Heat Transfer 8 (3) (1994) 563]. In addition, flow and heat transfer in a channel partially or fully filled with porous layers and containing solid protruding blocks with constant heat flux on its lower surface are also investigated in details. Hydrodynamic and heat transfer results are reported for both steady and transient flow cases. In particular, the effects of Darcy and Reynolds numbers on heat transfer augmentation and pressure loss are studied. An in-depth discussion of the formation and variation of recirculation is presented and the existence of optimum porous insert is demonstrated. At high Reynolds numbers the flow in the porous channel exhibits a cyclic characteristics although unlike the non-porous channel flow, the cyclic vortex development is only restricted to a small area behind the last solid block, while temperature changes more slowly and does not exhibit cyclic variations over a long period of time. It is shown that for all the cases studied altering some parametric values can have significant and interesting effects on both flow pattern as well as heat transfer characteristics.     

Oscillating plate temperature effects on a flow past an infinite vertical porous plate with constant suction and embedded in a porous medium

 An approximate solution to the problem of flow of a viscous incompressible dissipative fluid past an infinite vertical porous plate embedded in a porous medium is presented here. The plate temperature is assumed to be oscillating about a constant mean temperature. Mean velocity and mean temperature, the transient velocity and temperature profiles are shown graphically. The mean skin-friction and the mean rate of heat transfer are also shown graphically. The expressions for the amplitude and the phase of the skin-friction and the rate of heat transfer are derived and their numerical values are listed in Tables. The effects of different parameters governing the unsteady flow are discussed. Received on 23 November 1998