Heat transfer mechanism of a swirling impinging jet in a stagnation region

Heat transfer characteristics of a swirling impinging jet have been experimentally examined using a combined particle image velocimetry (PIV) and laser‐induced fluorescence (LIF) technique for simultaneous measurement of velocity and temperature fields. The present study shows that the radial width of the jet stretches with increasing swirl intensity, and that the stretching phenomenon contributes to the maximum local heat transfer coefficient. At the stagnation region, the flow near the heated surface is mixed intermittently by reverse flows toward upstream, and spatial distributions of temperature are correlated with instantaneous velocity vector maps. The dynamic behavior of recirculation zones, attributed to swirl number Sw and impinging distance, mainly determines the turbulent heat transfer at the stagnation region. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(8): 663–673, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10120     

A mass transfer study of the electropolishing of metals in mechanically agitated vessels

Rates of electropolishing of vertical copper plates in H₃PO₄ were studied in a rectangular cell stirred by 4 blade 45° pitched turbine. Variables studied were impeller rotating speed, plate height and H₃PO₄ concentration. The limiting current density at which polishing takes place was found to increase with impeller rotation speed to the power of 0.44, increasing the plate height (L) was found to decrease the limiting current density (I) according to the equation: I = a L^−0.24. The data were correlated under different conditions by the dimensionless equation Sh = 2.5 Sc^0.33 Re^0.44 (L/d_e)^−0.24     

Turbulent mass transfer in the mass transfer entry region for non-newtonian fluids

The effect of non-Newtonian flow behavior on turbulent mass transfer in the mass transfer entry region is investigated analytically. The predicted mass transfer rates in the mass transfer entry region are in good agreement with available experimental data and empirical correlations. The theoretical results for drag-reducing fluids show about 30 – 50% mass transfer rate reduction in the entry region which is less than that in the fully developed region.     

The skin friction in the MHD mixed convection stagnation point with mass transfer

Numerical results are presented for the effects of mass transfer on steady two-dimensional laminar MHD mixed convection owing to the stagnation flow against a heated vertical semi-infinite permeable surface. These results are obtained by solving the coupled nonlinear partial differential equations describing the conservation of mass, momentum and energy by a perturbation technique. These results are presented to illustrate the influence of the Hartmann number, wall mass transfer coefficient, heat absorption coefficient, Prandtl number and the mixed convection or buoyancy parameter. Numerical results for the dimensionless velocity profiles, the temperature profiles, the local friction coefficient and the local Nusselt number are presented for various parameters. These effects of the different parameters on the velocity and temperature as well as the skin friction and wall heat transfer are presented graphically.     

Dual solutions in a double-diffusive convection near stagnation point region over a stretching vertical surface

The development of double-diffusive convection near stagnation point region over a stretching vertical surface with constant wall temperature has been investigated. The external flow and the stretching velocities are assumed to vary with $\sqrt{x}\text{,}$ where x is the distance from the slot where the stretching surface is issued. Using the local similarity method, it has been shown that a set of suitable similarity transformations reduces the non-linear coupled partial differential equations governing the flow, thermal and concentration fields into a set of non-linear coupled ordinary differential equations. The non-linear self-similar equations along with the boundary conditions form a two point boundary value problem and are solved using Shooting method, by converting into an initial value problem. In this method, the system of equations is converted into the set of first order system which is solved by fourth-order Runge–Kutta method. Flows with both assisting and opposing buoyancy forces are considered in the present investigation. The study reveals that the dual solutions of velocity, temperature and concentration exist for certain values of suction/injection and buoyancy parameters. Prandtl and Schmidt numbers strongly affect the thermal and concentration boundary layer thicknesses, respectively. The effects of various parameters on the velocity, temperature and concentration profiles are also presented here.     

Effects of mass transfer on the stagnation point flow of an upper-convected Maxwell (UCM) fluid

This article deals with the effects of mass transfer on the two-dimensional stagnation point flow of an upper-convected Maxwell (UCM) fluid over a stretching surface. The similarity transformations convert the governing nonlinear partial differential equation into nonlinear ordinary differential equation. Computations for the outcoming systems are presented by a homotopy analysis method (HAM). Graphical results for the velocity and concentration fields are sketched and discussed.     

Laminar mass transfer in the wake region of closely spaced drops

A new model for mass transfer in the laminar wake region formed by closely spaced drops is proposed. The wake mass transfer model can be used to calculate the time-dependent local ambient conditions experienced by interacting drops falling on a verticle trajectory. Results are shown for a representative three and five-drop streamer absorbing a trace gas from a stagnant environment.     

Numerical simulation of heat and mass transfer in metal hydride hydrogen storage tanks for fuel cell vehicles

This paper presents a two-dimensional mathematical model to optimized heat and mass transfer in metal hydride storage tanks (hereinafter MHSTs) for fuel cell vehicles, equipped with finned spiral tube heat exchangers. This model which considers complex heat and mass transfer was numerically solved and validated by comparison with experimental data and a good agreement is obtained.     

Symmetry analysis of MHD Casson fluid flow for heat and mass transfer near a stagnation point over a linearly stretching sheet with variable viscosity and thermal conductivity

In this article, the impacts of variable viscosity and thermal conductivity on magnetohydrodynamic, heat transfer, and mass transfer flow of a Casson fluid are analyzed on a linearly stretching sheet inserted in a permeable medium along with heat source/sink and viscous dissipation. To reduce the ascendant partial differential equations into ordinary differential equations, Lie group transformation is utilized. Further, the fourth-order Runge–Kutta strategy is utilized to solve the ordinary differential equations numerically. The numerical results obtained for various parameters by employing coding in MATLAB programming are investigated and considered through graphical representation and tables. We anatomize the impacts of distinctive parameters on velocity, temperature, and concentration distributions.     

Heat and mass transfer in composite desiccant pore structures for dehumidification

A composite desiccant dehumidifier made of mixed inert and desiccant materials is investigated. A heat and mass transfer model that incorporates the composite nature of the structure is discussed. The model includes both gas-side and solid-side resistances for heat and mass transport. The solid-side resistance for the mass transport includes gas-phase diffusion and surface diffusion. The effectiveness for moisture removal and heat transfer during adsorption and desorption processes in such a dehumidifier during single blow operation is investigated. Results are presented for composite structures made of silica gel and inert materials of different compositions and thermophysical properties.     

Alterations to coherent flow structures and heat transfer due to pulsations in an impinging air-jet

An investigation was carried out to study the effect of flow pulsation on the characteristics of a planar air jet impinging normally on a heated surface. Such information was further utilized to determine the influence of flow characteristics in the plane of impingement on Nusselt number distribution. Time-resolved system properties were investigated with modern instrumentation that allowed instantaneous heat transfer and flow velocity measurements to be performed simultaneously. Based on good coherence function estimates between the signals, heat transfer measurements were used in return to infer flow dynamics near the impingement surface. Experiments were performed for steady and pulsating jets at jet Reynolds numbers of 1 000, 5 500, and 11 000, pulse frequencies up to 82 Hz (corresponding to Strouhal numbers below 0.13), and pulse amplitude at the nozzle exit up to 50 % of the mean flow velocity. Special techniques commonly used for periodically disturbed flow fields elucidated the dynamics of the pulse and associated coherent flow structures. Results indicated the parametric conditions for which alterations are expected in time-averaged heat transfer from the surface. Engineering applications include cooling of electronic packages and heat transfer to gas turbine blades.     

Numerical analysis of heat and mass transfer during absorption of hydrogen in metal hydride based hydrogen storage tanks

In this paper, hydriding in a cylindrical metal hydride hydrogen storage tank containing HWT5800 (Ti_0.98Zr_0.02V_0.43Fe_0.09Cr_0.05Mn_1.5) is numerically studied with a two-dimensional mathematical model. The heat and mass transfer of this model is computed by finite difference method. The effects of supply pressure, cooling fluid temperature, overall heat transfer coefficient and height to the radius ratio of the tank (H/R) on the hydriding in the hydrogen storage tank are studied. It is found that hydride formation initially takes place uniformly all over the bed and hydriding processes take place at a slower rate at the core region. Supply pressure, cooling fluid temperature and overall heat transfer coefficient play significant roles during the absorption of hydrogen. At the H/R = 2 both maximum bed temperature and the average bed temperature are the highest, and the hydride bed takes the longest time to saturate.     

Heat and mass transfer in porous media with ice inclusions near the freezing-point

Heat and mass transfer in biporous medium of regular structure near the phase transition point is studied theoretically. Large pores contain ice. Small pores are filled with pure water. The thermal and filtration problems for a separate cell of the medium are solved by the anisotropic conductivity method. Heat and mass flows depend linearly on the gradients of the temperature and the water pressure. The Onsager reciprocal relations are confirmed for systems with phase transformations. With the advent of the solid phase (ice) in porous media, the straight transport coefficients multiply several times, and the cross coefficients increase more than one order of magnitude.     

Transient convective heat transfer to water near the critical region in a vertical tube

Numerical analysis has been carried out to investigate transient forced convective heat transfer to water near the critical region in developing flow through a vertical tube. With large variations of thermophysical properties such as density, specific heat, viscosity, and thermal conductivity near the thermodynamic critical point, heat transfer in the tube is strongly coupled with fluid flow. Buoyancy force parameter has also large variation with fluid temperature and pressure in the tube. Time dependent characteristics of fluid velocity, temperature, and heat transfer coefficient with water properties are presented and analyzed. Transient Nusselt and Stanton number distributions along the tube are also compared for various pressures in the tube. Because of heat transfer from the wall transition behavior from liquid-like phase to gas-like phase of heat transfer coefficient occurs when the fluid passes through pseudocritical temperature region in the tube. Turbulent viscosity ratio also has steep variation near the pseudocritical temperature close to the wall.     

Large eddy simulations of wall heat transfer and coherent structures in mixed convection over a wavy wall

In this numerical study the mixed convective flow of water over a heated wavy surface over a range of Reynolds and Richardson numbers, including transitional and turbulent flow regimes (20 ≤ Re ≤ 2000 and 0.5 ≤ Ri ≤ 5000) is investigated. A dynamic Large Eddy Simulation (LES) approach is applied where the thermal buoyancy effects are represented by the Boussinesq approximation. The LES results show good agreement with available measurements including first and second order statistics of velocity and thermal fields. We focus our investigation on the thermal buoyancy effects on the wall heat transfer and the spatial reorganisation of the vortical flow structures. In order to characterise the reorganisation of the mean flow features, the vortical coherent structures are identified and extracted according to the swirling strength criteria. Interesting reorganisation of flow structures takes place between Re = 20 and Re = 200 where the initially spanwise oriented large coherent structures start to be streamwise oriented. With further increase of Re, these large structures disappear from the central part of the simulated domain and reappear in the proximity of the horizontal wavy wall for Re ≥ 1000. The imprints of this flow reorganisation are clearly visible in the distributions of the local heat transfer coefficient (Nusselt number) along the horizontal wavy wall. The integral heat transfer for the wavy wall configuration is significantly enhanced (≈2.5 times) for Re = 1000, 2000 in comparison with the standard flat horizontal wall configuration.