Evaporation heat transfer and pressure drop characteristics of r-134a in the oblong shell and plate heat exchanger

The evaporation heat transfer coefficienthr and frictional pressure drop δp_f of refrigerant R-134a flowing in the oblong shell and plate heat exchanger were investigated experimentally in this study. Four vertical counterflow channels were formed in the oblong shell and plate heat exchanger by four plates of geometry with a corrugated sinusoid shape of a 45° chevron angle. Upflow of refrigerant R-134a boils in two channels receiving heat from downflow of hot water in other channels. The effects of the refrigerant mass flux, average heat flux, refrigerant saturation temperature and vapor quality of R- 134a were explored in detail. Similar to the case of a plate heat exchanger, even at a very low Reynolds number, the flow in the oblong shell and plate heat exchanger remains turbulent. The results indicate that the evaporation heat transfer coefficienthr and pressure drop Δp_f increase with the vapor quality. A rise in the refrigerant mass flux causes an increase in theh _r and Δp_f. But the effect of the average heat flux does not show significant effect on the h_r and Δp_f. Finally, at a higher saturation temperature, both theh _r and Δp_f are found to be lower. The empirical correlations are also provided for the measured heat transfer coefficient and pressure drop in terms of the Nusselt number and friction factor.     

Effect of variable viscosity on thermal boundary layer over a permeable flat plate with radiation and a convective surface boundary condition

In this paper, the combined effects of radiation, temperature dependent viscosity, suction and injection on thermal boundary layer over a permeable flat plate with a convective heat exchange at the surface are investigated. By taking suitable similarity variables, the governing boundary layer equations are transformed into a boundary value problem of coupled nonlinear ordinary differential equations and solved numerically using the shooting technique with sixth-order Runge-Kutta integration scheme. The solutions for the velocity and temperature distributions together with the skin friction coefficient and Nusselt number depend on six parameters; Prandtl number Pr, Brinkmann number Br, the radiation parameter Ra, the viscosity variation parameter a, suction/injection parameter f _w and convection Biot number Bi. Numerical results are presented both in tabular and graphical forms illustrating the effects of these parameters on thermal boundary layer. The thermal boundary layer thickens with a rise in the local temperature as the viscous dissipation, wall injection, and convective heating each intensifies, but decreases with increasing suction and thermal radiation. For fixed Pr, Ra, Br and Bi, both the skin friction coefficient and the Nusselt number increase with a decrease in fluid viscosity and an increase in suction. A comparison with previously published results on special case of the problem shows excellent agreement.     

Combined free and forced convection flow of an elasto-viscous liquid through a porous channel

The combined effect of free and forced convection on the flow of an elasto-viscous liquid between two porous parallel plates with suction and injection at the walls has been studied. The effect of dimensionless numbers such as the elastic number Rc, the cross flow Reynolds number R, the Grashof number G, the Prandtl number Pr, the Brinkman number K and the wall temperature parameter N on the velocity and temperature fields, shear stresses and the rates of heat transfer at the walls have been studied.     

Entropy generation in hydrodynamic slip flow over a vertical plate with convective boundary

The present article aims to report the effects of hydrodynamic slip on entropy generation in the boundary layer flow over a vertical surface with convective boundary condition. Suitable similarity transformations are used to transform the fundamental equations of hydrodynamic and thermal boundary layer flow into ordinary differential equations. The governing equations are then solved numerically using the shooting method and the velocity and the temperature profiles are obtained for various values of parameters involved in the governing equations. The expressions for the entropy generation number and the Bejan number are presented and the results are discussed graphically and quantitatively for the slip parameter, the local Grashof number, the Prandtl number, the local convective heat transfer parameter, the group parameter and the local Reynolds number. It is observed that due to the presence of slip, entropy production in a thermal system can be controlled and reduced.     

A numerical study on mixed convection in a lid-driven cavity with a circular cylinder

A two-dimensional numerical simulation is carried out in this study to investigate mixed convection in a lid-driven cavity with an isothermal circular cylinder. The simulation is conducted at three Reynolds numbers of Re = 100, 500, and 1000 under a fixed Grashof number of Gr = 10⁵. The top wall of the cavity moves to the right at a constant velocity and is kept at a low temperature of T _c , whereas the stationary bottom wall is kept at a constant high temperature of T _h . The immersed-boundary method, which is based on the finite volume method, is adopted for the boundary of the circular cylinder that is present in the square cavity. The present study aims to investigate the effects of circular cylinder on fluid flow and heat transfer in a cavity at different locations. The fluid flow and heat transfer characteristics in the cavity strongly depend on the position of the circular cylinder as well as on the relative magnitude of the forced convection and the natural convection caused by the movement in the top wall of the cavity and the heating at the hot bottom wall, respectively.     

Nanofluid thin film flow and heat transfer over an unsteady stretching elastic sheet by LSM

This study is carried out on the unsteady flow and heat transfer of a nanofluid in a stretching flat plate. Least square method is implemented for solving the governing equations. It also attempts to demonstrate the accuracy of the aforementioned method compared with a numerical one, Runge-Kutta fourth order. Furthermore, the impact of some physical parameters like unsteadiness parameter (S), Prandtl number (Pr) and the nanoparticles volume fraction (?) on the temperature and velocity profiles is scrutinized carefully. Accordingly, the results obtained from this study reveal that the temperature enhances by means of augmenting the nanoparticles volume fraction. At η ∈ {0, 0.5}, the velocity decreases as a result of a rise in nanoparticles volume fraction and at η ∈ {0.5, 1}, an opposite treatment takes place. Moreover, velocity distribution augments by raising the S value, however an inverse trend is observed in temperature values. Moreover, the local skin friction coefficient indicated a notable rise by increasing the S parameter as well as a steady decrease by rising ?. Finally, water-Alumina nanofluid demonstrated better heat transfer enhancement compared to other types of nanofluids.     

Analysis of enhancement and impairment mechanisms of natural convection heat transfer on a vertical finned plate

We investigated the heat transfer enhancement and impairment mechanisms of the laminar natural convection on a vertical finned plate. Numerical analyses were performed for wide ranges of Prandtl numbers 0.7–2014, Rayleigh numbers 3.69×10⁵−8.49×10¹⁰ and fin heights 0.0025–0.5 m. Experiments were performed for a few cases for verification. Four different heat transfer mechanisms were identified: corner, core acceleration, chimney and in-flow effects. The competitions of these mechanisms depending on the fin geometries and the Prandtl number resulted in complex variations of the heat transfer. The results showed the heat transfer enhancement of maximum 6.9 % for Pr = 2014, L = 0.1 m and H = 0.015 m and impairment up to 47 % for Pr = 0.7, L = 0.1 m and H = 0.015 m compared with that of a flat plate with the same heat transfer area and baseplate length.

     

A turbulent friction and heat transfer study of dispersed fluids with ultra-micronized metallic particles

Turbulent friction and heat transfer behaviors of dispersed fluids with ultra-micronized metallic particles are experimentally investigated in a circular pipe. Viscosity measurements are also conducted by using a viscometer. Aqueous mixtures with γ-Al₂O₃ and TiO₂ particles of which the mean diameters are 13 and 27 nm, respectively, are used to represent the dispersed fluids. The ranges of Reynolds and Prandtl numbers tested are 10⁴～10⁵ and 5.6～10.7, respectively. The relative viscosity of the dispersed fluid with γ-Al₂O₃ particles is about two hundred at the 10% volume concentration, while that of the dispersed fluid with TiO₂ particles is about twenty at the same volume concentration. Both of the relative viscosities are the unexpected results compared with predictions from classical theory of suspension rheology. Darcy friction factors for the comparatively dilute dispersion fluids used in present study coincide well with Kays correlation for tubulent flow of a single phase fluid, which implies that additional pumping power is not required despite adding solid particles into water. The Nusselt number of both the dispersed fluids for fully developed turbulent flow increases with increasing the volume concentration as well as the Reynolds number as expected. At the maximum volume concentration of 3% approximately, the percentage heat transfer enhancement due to addition of particles for the γ-Al₂O₃ and TiO₂ dispersing fluid systems are 60% and 30%, respectively. Under the range of volume concentration in the present study, the new correlation for turbulent convective heat transfer for both of the dispersed fluids is given by the following equation: Nu=0.021Re^0.8Pr.^0.5     

Investigation of a novel Couette flow with cylindrical baffles

The high-precision measure instrument for flow velocity is essential for industrial applications because the high-precision velocity can well reflect the physical characteristic of the flow. A restricted laminar Couette flow with cylindrical baffles, using a synthetic heat conduction liquid, was designed to obtain a steady vortex flow and wider work scope, according to Couette flow and Suspension flow characteristics. The heat transfer mechanism was investigated with a laminar flow model by the Fourier law. The research indicates that the heat transfer enhancement is related to the Temperature Boundary Layer (TBL). The TBL is affected by the Velocity Boundary Layer (VBL). The TBL thickness and Nusselt number (Nu) have a dependent relationship. The Reynolds number (Re) and the gap between the baffle and plate wall (Δh/h) can further affect Nu. The vortex flow generated by Couette flow can significantly enhance the heat transfer performance by a double spiral structure, which can rapidly mix heat fluxes and make the temperature converge to uniform. There is a sensitive and stable relationship between flow velocity and heat transfer. Notably, it is linear when Δh/h or Re is small, which can be used to design a high-precision thermal flow velocity meter.     

Analysis of mixed convection in an inclined lid-driven cavity with a wavy wall

A numerical study is performed to analyze the mixed convection flow and heat transfer in a lid-driven cavity with sinusoidal wavy bottom surface. The cavity vertical walls are insulated while the wavy bottom surface is maintained at a uniform temperature higher than the top lid. A finite volume method is used to solve numerically the non-dimensional governing equations. The tests were carried out for various inclination angles ranging to 0° from 180° and number of undulation varied from 4 to 6, while the Prandtl number was kept constant Pr = 0.71. Three geometrical configurations were used namely four, five and six. The distributions of streamlines and isotherms, and the variations of local and average Nusselt numbers with the inclination angle are presented. The results of this investigation illustrate that the average Nusselt number at the heated surface increases with an increase of the number of undulations as well as the angle of inclination.     

Numerical investigation of effects of buoyancy around a heated circular cylinder in parallel and contra flow

Two-dimensional, steady, incompressible Navier-Stokes and energy equations are expressed in the stream function/vorticity formulation and solved numerically by finite difference method to study effects of buoyancy on fluid flow and heat transfer from a horizontal circular cylinder. The cylinder is exposed to approaching flow stream, for parallel (parallel flow) and opposing (contra flow) directions to the buoyant force. Two different thermal boundary conditions were considered at the cylinder surface: constant temperature (CT) and constant heat flux (CHF). The results elucidating the dependence of the flow and heat transfer characteristics on the Richardson number 0≤ Ri ≤ 2, Prandtl number 0 ≤ Pr ≤ 100 and Reynolds number 0 ≤ Re ≤ 40 are presented. Overall, for parallel flow regime, an increase in the Ri led to a raise in both Nusselt number and drag coefficient. However, for contra flow regime, these trends were reversed. For both regimes, the aforementioned behaviors were more pronounced for CT boundary condition than that for the CHF boundary condition.     

Further results on non-Newtonian power-law flows past a two-dimensional flat plate with finite length

The flow of a non-Newtonian, power-law fluid directed either tangentially or normally to a flat plate of finite length and infinite width (two-dimensional flow) is considered. The problem is investigated numerically using the code ANSYS FLUENT. This problem has been investigated in the past but only for shear-thinning fluids (n < 1). We extend the investigation for the case of shear-thinning, Newtonian and shear-thickening fluids, covering a wide range of Reynolds numbers (from very low to very high). For low Reynolds numbers and low power-law index (n < 0.6) the drag coefficient obeys the relationship c _D = A/Re, both for tangential and normal flow. Equations for the quantity A have been derived as functions of the power-law index. For normal flow, the drag coefficient tends to become independent of the power-law index, both for shear-thinning and shear-thickening fluids at high Reynolds numbers.     

Modeling of laminar forced convection in spherical-pebble packed beds

There are many parameters that have significant effects on forced convection heat transfer in packed beds, including Reynolds and Prandtl numbers of flow, porosity, pebble geometry, local flow conditions, wall and end effects. In addition, there have been many experimental investigations on forced convection heat transfer in packed beds and each have studied the effect of some of these parameters. Yet, there is not a reliable correlation that includes the effect of main parameters; at the same time, the prediction of precise correct limits for very low and high Reynolds numbers is off hand. In this article a general well-known model of convection heat transfer from isothermal bodies, next to some previous reliable experimental data has been used as a basis for a more comprehensive and accurate correlation to calculate the laminar constant temperature pebble-fluid forced convection heat transfer in a homogeneous saturated bed with spherical pebbles. Finally, for corroboration, the present results are compared with previous works and show a very good agreement for laminar flows at any Prandtl number and all porosities.     

Heat transfer in a turbulent jet impinging on a moving plate considering high plate-to-jet velocity ratios

In this paper, heat transfer characteristics of a turbulent slot jet impinging orthogonally on an isothermal moving hot plate is studied numerically. The governing equations were discretized using the finite volume method and the υ ² — f turbulence model was employed for turbulence modeling. The effect of the jet Reynolds number and the plate-to-jet velocity ratio (R) on the Nusselt were investigated. Despite of most previous studies, which have been restricted to R≤2, in the present research higher values of R, also were considered (0≤R≤6). Range of studied jet Reynolds number was between 3000 and 60000. The results indicate that at a fixed plate-to-jet velocity ratio increment of the Reynolds number leads to the enhancement of the average Nusselt number. For each Reynolds number, the average Nusselt number reduces with increasing the plate-to-jet velocity ratio until it becomes minimum at R = 1.25. For R>1.25 trend changes so that these parameters increase. In addition, it was found that only for R>2.5 the average Nusselt number is improved due to the plate motion in comparison with the stationary jet. The results are validated against available experimental data, showing good agreement.     

Intensified heat transfer in pipe with tall baffles

A model of the heat transfer in pipe with tall baffles is proposed, in conditions of intensified heat transfer. The model takes account of various factors for a broad range of Prandtl numbers and Reynolds numbers.     

Analysis of contoured holes produced using STED process

Recent developments in air engines call for more efficient means of turbine blade cooling to have higher power generation for the same unit size with increased inlet air temperature. To allow the turbine to operate at higher temperature, thousands of cooling holes are drilled in turbine blades. In order to increase heat transfer in cooling holes, the present design demands the wall of the cooling passage should be provided with contoured ribs. These irregularities help in inducing turbulence in the flow of cooling air, thereby increasing the rate of heat transfer. For drilling these kinds of contoured deep holes in a turbine blade made of material such as Inconel, the conventional drilling techniques are not suitable. The shaped tube electrolytic drilling (STED) is used to perform the task of drilling contoured holes in difficult-to-machine materials. In the present case, contoured holes are drilled by using two distinct feed rates f ₁ and f ₂ alternately (f ₁ > f ₂) and two types of workpiece materials, namely stainless steel and Inconel superalloy. Experimentally obtained profiles are compared with the profiles derived theoretically from the basic electrochemical machining equations. Quality performance factor is evaluated for the machined holes to find the best hole profile. From the present study, it has been observed that by varying the process parameters (viz., voltage (V), faster feed rate (f ₁), and slower feed rate (f ₂)) during drilling and fixing different step lengths, various types of hole profiles can be generated using STED process.     

On the large eddy simulation of high prandtl number scalar transport using dynamic subgrid-scale model

The present study investigates passive scalar transport using an eddy viscosity/diffusivity model in turbulent channel flow with Prandtl number range 1-10. Dynamic subgrid-scale model (DSM) was applied to the transport equation for passive scalar to determine the eddy diffusivity dynamically. To assess the feasibility of the DSM model applied for passive scalar,a priori test on direct numerical simulation data was conducted and the results are compared with those obtained from a large eddy simulation that uses DSM modela posteriori. As the Prandtl number increases, the discrepancy in subgrid-scale (SGS) heat flux amplifies but the shape of SGS temperature dissipation profiles shows reasonable agreement. This suggests that energy transfer between resolved and subgrid-scales are reasonably predicted regardless of the accuracy in SGS heat flux vectors. Whilea priori test shows that SGS turbulent Prandtl number changes significantly with Prandtl number, the actual LES results are found to be insensitive to Prandtl number away from the wall. Thus, the DSM model has some limitations in the prediction of high Prandtl number flows.     

The effects of the compressibility of the subsonic flow on heat transfer characteristics in a microscale impinging slot jet

We experimentally investigated the effects of both the compressibility and nozzle width on the local heat transfer distribution of microscale unconfmed slot jets impinging on a uniformly heated flat plate. We made heat transfer measurements under the following experimental conditions; Reynolds numbers of Re = 4000~10000, Mach numbers of Ma = 0.13~0.68, nozzle-to-plate distances of H/B = 3~25, lateral distances of x/B = 0~25, and nozzle widths of B = 300~700 μm having a nozzle aspect ratio of y/B = 30. A thermal infrared imaging technique was used to measure the impingement plate temperature. The experimental results show that for all tested Re and H/B values at a nozzle width of B = 300 μm, the Nusselt number maximum occurred nearly at the stagnation point and then monotonically decreased along the downstream. However, at B = 500 and 700 μm, the maximum Nusselt number point shifted toward x/B ≈ 1.5~2.0. And the Nusselt number increased, as x/B increased, from the stagnation point to the shifted maximum point and monotonically decreased afterward. This shifted maximum point may be attributable to vortex rings promoting sudden flow acceleration and entrainment of surrounding air moving along the jet axis. For the same Reynolds number, the Nusselt number in the stagnation region increased as the nozzle width increased due to a momentum increase of the jet flow caused by the formation of vortices. And, the Nusselt numbers for the smallest nozzle width of B = 300 μm (or highest Mach number at a given Reynolds number) at all H/B and Reynolds numbers tested significantly deviated from those for B = 500 and 700 μm in the downstream region corresponding to x/B > 5, suggesting that the compressibility, when it is high, can affect the heat transfer in the downstream region.

     

Analytic heat and mass transfer of the mixed hydrodynamic/thermal slip MHD viscous flow over a stretching sheet

In this paper, the heat and mass transfer characteristics of the magnetohydrodynamic (MHD) viscous flow over a permeable stretching surface is solved analytically. The flow considered is under both the hydrodynamic and thermal slip conditions. The magnetohydrodynamic flow and heat transfer of an electrically conducting fluid, taking into account the effects of Joule and viscous dissipation, internal heat generation/absorption, work done due to deformation and thermal radiation is studied. The solution is expressed in a closed form equation and is an exact solution of the full governing Navier-Stokes and energy equations. Thermal transport is analyzed for two types of non-isothermal boundary conditions, i.e. prescribed surface temperature (PST) and prescribed surface heat flux (PHF) varying as a power of the distance from the origin. Results for some special cases of the present analysis are in excellent agreement with those existing in the literature. The effects of various physical parameters, such as magnetic parameter, thermal radiation parameter, heat source/sink parameter, Prandtl number, Eckert number and suction/injection parameter on the velocity and temperature profiles, skin friction coefficient and Nusselt number are examined and discussed in detail. Results show that there is only one physical solution for any combination of the slip together with all the parameters. The velocity/shear stress profiles and the temperature/heat transfer profiles are greatly influenced by these parameters.     

Experimental and numerical study of heat transfer downstream of an axisymmetric abrupt expansion and in a cavity of a circular tube

This research is an experimental and numerical investigation of heat transfer and fluid flow characteristics in separated, recirculated and reattached regions created by an axisymmetric abrupt expansion and by an abrupt expansion followed by an abrupt contraction (called a “cavity”) in a circular tube at a uniform wall temperature. The flow just upstream of the expansion was unheated and proved to be fully-developed at the entrance to the heated cavity region. Local heat transfer coefficients were measured using a balance-type isothermal heat flux gage. Measurements were made at a small-to-large tube diameter ratio of d/D = 0.4 and downstream Reynolds numbers ranging from Re_D = 4,300 to 44,500. Generally, the maximum Nusselt numbers downstream of an axisymmetric abrupt expansion at a uniform wall temperature occur between 9 and 12 step heights from the expansion step. Numerical simulation has been carried out by a two-equation turbulence model and its results such as mean velocity profiles and local Nusselt numbers are in good agreement with experimental results.