Local turbulent opposing mixed convection heat transfer in inclined flat channel for unstably stratified airflow

Local turbulent mixed convection heat transfer in inclined (from ? = 0° (horizontal position) till ? = 90° (vertical position)) flat channels for opposing flows was investigated for the case when only bottom wall is heated (unstably stratified flow conditions). Wide ranges of airflow parameters are covered: Re = 4 × 10³-6.6 × 10⁴, Gr_q = 4.7 × 10⁷-6.3 × 10¹⁰, pressure p = 0.1; 0.2; 0.4; 0.7; 1.0 MPa. Correlation for calculation of heat transfer in inclined flat channels was suggested for the region without buoyancy instabilities. The experimental data were compared with the recent experimental data for inclined flat channels when upper wall is heated (stably stratified flow conditions).     

Unsteady turbulent heat transfer of mixed convection in a reciprocating circular ribbed channel

The SIMPLE-C scheme is used to solve the mass, momentum, energy conservation equations and turbulent k-ε equations with a two-layer model near wall for a fluid past a reciprocating circular ribbed channel when changing Reynolds number (4250-10,000), Grashof number (0-400,000,000), pulsating number (0-9.3) and cooling mediums. The average time-mean Nusselt number for the reciprocating circular ribbed channel can be 45-182% larger than that for the equivalent stationary smooth channel. The heat transfer enhancement produced by buoyancy for the reciprocating circular ribbed channel decreases as the pulsating number increases. The oscillating amplitude of Nusselt number with crank angle in the oil-cooling is less than in the water-cooling.     

Upflow turbulent mixed convection heat transfer in vertical pipes

The present work deals with the results of an experimental investigation on heat transfer in water cooled vertical pipes, for thermal–hydraulic conditions ranging from forced convective flow to mixed convective flow. The flow of water in the pipe is upwards.Experimental data confirm the reduction in the heat transfer rate for mixed convection in upward heat flow, mainly due to the laminarization effect in the near-wall region (buoyancy effect) . They are in a very good agreement with numerical methods, such as the k–-model.A new method for the calculation of the heat transfer coefficient in upward mixed convection heated flow is proposed. It is based on the well-known superposition method (heated downflow) modified accounting for the phenomenology of the upward heated flow in comparison with downflow heated conditions.     

Numerical simulation of conjugate,turbulent mixed convection heat transfer in a vertical channel with discrete heat sources

This paper presents the results of a comprehensive numerical study to analyze turbulent mixed convection in a vertical channel with a flush-mounted discrete heat source in each channel wall. The conjugate heat transfer problem is solved to study the effect of various parameters like the thermal conductivity of the wall material (k_s), the thermal conductivity of the flush-mounted discrete heat source (k_c), Reynolds number (Re_s), modified Richardson number (Ri⁎) and the aspect ratio of the channel (AR). The standard k–ε turbulence model, modified by including buoyancy effects, without wall functions, has been used for the analysis. The two-dimensional governing equations are discretised on a semi-staggered, non-uniform grid, using the finite volume method. The asymptotic computational fluid dynamics (ACFD) technique has been then applied to obtain a correlation for the non-dimensional maximum temperature θ¯_max, which can be used for a wide range of parameters.     

Fluid flow and heat transfer of opposing mixed convection adjacent to upward‐facing,inclined heated plates

Opposing mixed convective flows of air induced over uniformly heated, upward‐facing, inclined plates were investigated experimentally. The experiments covered the ranges of the Reynolds and modified Rayleigh numbers, Re_L=7.2×10² to 1×10⁴ and Ra_L^*=5×10⁶ to 8×10⁸, and the inclination angles from θ=15 to 75^° from horizontal. The flow fields over plates were visualized with smoke. The results showed that a separation of forced boundary layer flow occurs first at the trailing edge, and then the separation point shifts toward upstream with increasing the wall heat flux, and finally, reaches to the leading edge of the plates. It was also found that the separations at the trailing and leading edges are correlated well with the non‐dimensional parameter as (Gr_θ^*/Re_L^2.5)=0.35 and 1.0, respectively. The local heat transfer coefficients of the inclined plates were also measured and the results showed that the above separation retards the heat transfer significantly from that of the forced convection. It was also revealed that the heat transfer by forced, natural, and combined convections can be classified with the above parameter as (Gr_θ^*/Re_L^2.5)<0.2,(Gr_θ^*)/Re_L^2.5)>3, and 0.2<(Gr_θ^*/Re_L^2.5)<3, respectively. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(3): 127– 142, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20151     

Fluid flow and heat transfer of opposing mixed convection adjacent to downward‐facing,inclined heated plates

Experimental investigations were carried out for opposing mixed convective flows of air adjacent to downward‐facing, inclined heated plates. The experiments covered the ranges of the Reynolds and modified Rayleigh numbers from Re_L=400 to 4600 and Ra_L^*=1.0×10⁷ to 5.4×10⁸, and the inclination angles from θ=15 to 75^° from horizontal. The flow fields over the plates were visualized with smoke. The results showed that a separation of forced boundary layer flow occurs first at the bottom edge of the plate, and then the separation point shifts toward upstream with increasing wall heat flux, and finally, reaches the top edge of the plates. It was found that the separations at the bottom and top edges are predicted with a non‐dimensional parameter (Gr_Lθ^*/Re_L^2.5)=0.35 and 1.0, respectively. The local heat transfer coefficients of the inclined plates were also measured and the results showed that the minimum coefficients appear in the separation region. Moreover, it was revealed that forced, natural, and combined convective flows can be classified by the non‐dimensional parameter (Gr_Lθ^*/Re_L^2.5). © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Pub‐ lished online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20233     

Parametric study on mixed convection heat transfer in an inclined arc-shape cavity

This paper presents a parametric study on mixed convection heat transfer in an inclined arc-shape cavity subjected to a moving lid. The governing equations for the inclined arc-shape cavity were derived with the incorporation of inertia and buoyant force terms and solved by using the finite-volume method and numerical grid generation scheme. The parametric study considered three physical parameters including inclination angle, Reynolds number and Grashof number, and explored the effect of these parameters on the flow field and heat transfer characteristics. Computations were conducted for the Reynolds number ranging from 100 to 1500, Grashof number from 10⁵ to 10⁷ and inclination angle from 15⁰ to 60⁰. The numerical results show that the flow pattern becomes inertia-dominant and the strength of the primary vortex generally increases as the Reynlods number increases. As the Grashof number increases, the strength of the inertial-induced vortex decreases and the strength of the buoyancy-induced vortex increases. The strength of the vortexes decreases with the increasing inclination angle and the buoyancy-induced flow becomes more dominant. The average Nusselt number increases as the Grashof number increases for all the inclination angles studied here. The local friction increases with the increasing inclination angle, and becomes significant as the Grashof number increases.     

Time-periodic laminar mixed convection in an inclined channel

The steady-periodic regime of laminar mixed convection in an inclined channel is studied analytically, with the following boundary conditions: the temperature of one channel wall is stationary, while the temperature of the other wall is a sinusoidal function of time. Analytical expressions of the velocity field, of the temperature field, of the pressure drop, of the friction factors, as well as of the Nusselt number at any plane parallel to the walls are determined. It is found that, for every value of the Prandtl number greater than 0.277, there exists a resonance frequency which maximizes the amplitude of the friction factor oscillations at the unsteady-temperature wall. Moreover, for any plane which lies between the midplane of the channel and the unsteady-temperature wall, every value of the Prandtl number yields a resonance frequency which maximizes the amplitude of the Nusselt number oscillations.     

Fluid flow and heat transfer of opposing mixed convection adjacent to vertical heated plates

Opposing mixed convective flows induced around heated vertical plates were investigated experimentally. The experiments have been carried out with air and test plates of 100 mm and 200 mm long. The flow fields over the heated plates were visualized and the local heat transfer coefficients of the plates were measured for a wide range of Reynolds and Rayleigh numbers; Re_L = 7 × 10²−1.5 × 10⁴, Ra = 9 × 10⁶−8 × 10⁸. The visualization experiments showed that the separation of the boundary layer appears first at the trailing edge of the plate when the non‐dimensional parameter of (Gr/Re) = 0.35, and that the separation point reaches the leading edge when (Gr_L^*/Re_L^2.5) = 1.0. The heat transfer experiments showed that the above flow separation retards the heat transfer significantly from that of pure forced convection. It was also revealed that the flows over the heated plates can be categorized into forced, natural, and combined convection using the above parameter, (Gr/Re), as (Gr/Re) < 0.2, (Gr/Re) > 3 and 0.2 < (Gr/Re) < 3, respectively. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(8): 595–607, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20080     

Natural convection heat transfer along a vertical flat plate with a projection in the turbulent region

In the present study, the heat transfer coefficients occurring with a projection in the turbulent region of a vertical flat plate were measured experimentally for various projection heights in the range of 0 to 20 mm. The wall temperature and fluid flow fields were also visualized using a liquid crystal sheet and nylon 12 powder, respectively. The average and local Nusselt numbers reach 1.07 to 1.22 and 1.2 to 1.7 times those for pure turbulent natural convection, respectively. The maximum enhancement rates of heat transfer are attained at a position of 2.3 to 3.3 times the projection height from the upper projection surface toward the downstream, and these positions are in good agreement with those of the reattachment of the fluid flow and with centers of dark red regions in the liquid crystal. On the other hand, the heat transfer coefficients in the just upstream and downstream regions of the projection are small compared with those for no projection. By introducing the nondimensional parameter R, the present experimental results are rearranged quantitatively and effectively. © 2001 Scripta Technica, Heat Trans Asian Res, 30(3): 222–233, 2001     

Enhancement of heat transfer in a turbulent channel flow with insertion of a flat body

Experiments have been performed for turbulent channel flow obstructed with a flat body. The local heat transfer coefficient and the wall static pressure were measured on two kinds of flat bodies for which the trailing edge shape differed. The length of the body, the thickness of the body, and the distance between the wall and the body were changed in several steps. The total performance between heat transfer and pressure drop was estimated under conditions of equal pumping power. The total performance hardly changed, even if the trailing edge shape and length of the bodies were different. The averaged heat transfer coefficient increased with increasing thickness of the bodies. However, as the friction factor increased, the performance became worse. When a comparatively thin body was installed near the heating surface, good performance was obtained. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(4): 354–366, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10100     

Natural convection heat transfer in partially open inclined square cavities

A numerical study has been carried out on inclined partially open square cavities, which are formed by adiabatic walls and a partial opening. The surface of the wall inside the cavity facing the partial opening is isothermal. Steady-state heat transfer by laminar natural convection in a two dimensional partially open cavity is studied by numerically solving equations of mass, momentum and energy. Streamlines and isotherms are produced, heat and mass transfer is calculated. A parametric study is carried out using following parameters: Rayleigh number from 10³ to 10⁶, dimensionless aperture size from 0.25 to 0.75, aperture position at high, center and low, and inclination of the opening from 0° (facing upward) to 120° (facing 30° downward). It is found that the volume flow rate and Nusselt number are an increasing function of Rayleigh number, aperture size and generally aperture position. Other parameters being constant, Nusselt number is a non-linear function of the inclination angle. Depending on the application, heat transfer can be maximized or minimized by selecting appropriate parameters, namely aperture size, aperture position and inclination angle at a given operation Rayleigh number.     

Experimental studies of combined heat transfer in turbulent mixed convection fluid flows in double-skin-façades

The current contribution describes the experiments performed in an outdoor test stand for so called “double-skin-façades” at the Technical University of Munich. The purpose of the investigation was to determine the time and local averaged overall heat transfer coefficients for solar radiation augmented turbulent mixed convection flows in transparent vertical channels. The external plate of the vertical channel is formed with horizontally oriented ventilation gratings in the external glass façade, thus providing openings for the air circulation within the gap. In the course of the research, the distance between the external and the internal glass façade was measured for three gaps of 0.3, 0.6 and 0.9 m wide. Using the pressure compensation method, the effective mass flow rates for a given box-window, which is the smallest functional unit of all known double-skin-façade systems, was measured by means of a ventilation duct installed at the outlet of such a box-window. Finally, the experimental data was reduced in terms of an average Nusselt number as a function of an average Archimedes number for several gap distances.     

Thermal protection with liquid film in turbulent mixed convection channel flows

In this numerical study, a channel flow of turbulent mixed convection of heat and mass transfer with film evaporation has been conducted. The turbulent hot air flows downward of the vertical channel and is cooled by the laminar liquid film on both sides of the channel with thermally insulated walls. The effect of gas–liquid phase coupling, variable thermophysical properties and film vaporization are considered in the analysis. In the air stream, the k–ε turbulent model has been utilized to formulate the turbulent flow. Parameters used in this study are the mass flow rate of the liquid film B, Reynolds number Re, and the free stream temperature of the hot air T_o. Results show that the heat flux was dramatically increases due to the evaporation of liquid water film. The heat transfer increases as the mass flow rate of the liquid film decreases, while the Reynolds number and inlet temperature increase, and the influences of the Re and T_o are more significant than that of the liquid flow rate. It is also found that liquid film helps lowering the heat and mass transfer rate from the hot gas in the turbulent channel, especially at the downstream.     

Buoyancy-aided/opposed convection heat transfer for unsteady turbulent flow across a square cylinder in a vertical channel

The Large Eddy Simulation (LES) and SIMPLE-C method coupled with preconditioned conjugate gradient methods have been employed to study the effect of aiding/opposing buoyancy on the turbulent flow field and heat transfer across a square cylinder in a vertical channel. The level of wall-confinement (blockage ratio of 10%, 30% and 50%) was changed with a constant Reynolds number (5000) under various Richardson numbers (−1 to 1). With increasing blockage ratio, the buoyancy effect is becoming weaker on the Nusselt number for the square cylinder. The turbulent heat transfer past the square cylinder can be improved by increasing the blockage ratio.     

Experimental investigation of mixed convection heat transfer from longitudinal fins in a horizontal rectangular channel

Mixed convection heat transfer from longitudinal fins inside a horizontal channel has been investigated for a wide range of modified Rayleigh numbers and different fin heights and spacings. An experimental parametric study was made to investigate effects of fin spacing, fin height and magnitude of heat flux on mixed convection heat transfer from rectangular fin arrays heated from below in a horizontal channel. The optimum fin spacing to obtain maximum heat transfer has also been investigated. During the experiments constant heat flux boundary condition was realized and air was used as the working fluid. The velocity of fluid entering channel was kept nearly constant (0.15 ? w_in ? 0.16 m/s) using a flow rate control valve so that Reynolds number was always about Re = 1500. Experiments were conducted for modified Rayleigh numbers 3 × 10⁷ < Ra¹ < 8 × 10⁸ and Richardson number 0.4 < Ri < 5. Dimensionless fin spacing was varied from S/H = 0.04 to S/H = 0.018 and fin height was varied from H_f/H = 0.25 to H_f/H = 0.80. For mixed convection heat transfer, the results obtained from experimental study show that the optimum fin spacing which yields the maximum heat transfer is S = 8–9 mm and optimum fin spacing depends on the value of Ra¹.     

Mixed convection heat transfer in inclined rectangular ducts with radiation effects

A numerical study was carried out to investigate the radiation effect on the characteristics of the mixed convection fluid flow and heat transfer in inclined ducts. The three-dimensional Navier–Stokes equations and energy equation are solved simultaneously with the vorticity–velocity method. The integro-differential radiative transfer equation was solved by the discrete ordinates method. The effects of the thermal buoyancy and the radiative transfer on the distributions of the bulk fluid temperature, the friction factor and the Nusselt number are emphasized in detail. Results indicate that radiation effects have a considerable impact on the heat transfer and tend to reduce the thermal buoyancy effects. In addition, the development of the bulk fluid temperature is enhanced by the radiation effects.