Experimental investigation of natural convection heat transfer in horizontal and inclined annular fluid layers

In the present study, an experimental investigation of heat transfer and fluid flow characteristics of buoyancy-driven flow in horizontal and inclined annuli bounded by concentric tubes has been carried out. The annulus inner surface is maintained at high temperature by applying heat flux to the inner tube while the annulus outer surface is maintained at low temperature by circulating cooling water at high mass flow rate around the outer tube. The experiments were carried out at a wide range of Rayleigh number (5 × 10⁴ < Ra < 5 × 10⁵) for different annulus gap widths (L/D _o = 0.23, 0.3, and 0.37) and different inclination of the annulus (α = 0°, 30° and 60°). The results showed that: (1) increasing the annulus gap width strongly increases the heat transfer rate, (2) the heat transfer rate slightly decreases with increasing the inclination of the annulus from the horizontal, and (3) increasing Ra increases the heat transfer rate for any L/D _o and at any inclination. Correlations of the heat transfer enhancement due to buoyancy driven flow in an annulus has been developed in terms of Ra, L/D _o and α. The prediction of the correlation has been compared with the present and previous data and fair agreement was found.     

Experimental study on turbulent natural convection heat transfer in water with sub-millimeter-bubble injection

Using thermocouples and a particle tracking velocimetry technique, temperature and velocity measurements are conducted to investigate flow and heat transfer characteristics of turbulent natural convection from a vertical heated plate in water with sub-millimeter-bubble injection. Hydrogen-bubbles generated by the electrolysis of water are used as the sub-millimeter-bubbles. In the turbulent region, the heat transfer deterioration occurs for a bubble flow rate Q = 33 mm³/s, while the heat transfer enhancement occurs for Q = 56 mm³/s. Temperature and velocity measurements suggest that the former is caused by a delay of the transition due to the bubble-induced upward flow. On the other hand, the latter is mainly due to two factors: one is the enhancement of the rotation of eddies in the outer layer, and the other is the increase in the gradient of the streamwise liquid velocity at the heated wall. These are caused by bubbles, which are located in the inner layer, rising at high speed.     

Cut-off cooling velocity profiling inside a keyhole model using the Boubaker polynomials expansion scheme

The time dependent heating and cooling velocities are investigated in this paper. The temperature profile is found by using a keyhole approximation for the melted zone and solving the heat transfer equation. A polynomial expansion has been deployed to determine the cooling velocity during welding cut-off stage. The maximum cooling velocity has been estimated to be V _max ≈ 83°C s⁻¹.     

Buoyancy and cross flow effects on heat transfer of multiple impinging slot air jets cooling a flat plate at different orientations

The present article reports on heat transfer characteristics associated with multiple laminar impinging air jet cooling a hot flat plat at different orientations. The work aims to study the interactions of the effects of cross flow, buoyancy induced flow, orientation of the hot surface with respect to gravity, Reynolds numbers and Rayleigh numbers on heat transfer characteristics. Experiments have been carried out for different values of jet Reynolds number, Rayleigh number and cross flow strength and at different orientations of the air jet with respect to the target hot plate. In general, the effective cooling of the plate has been observed to be increased with increasing Reynolds number and Rayleigh number. The results concluded that the hot surface orientation is important for optimum performance in practical applications. It was found that for Re ≥ 400 and Ra ≥ 10,000 (these ranges give 0.0142 ≤ Ri ≤ 1.59 the Nusselt number is independent on the hot surface orientation. However, for Re ≤ 300 and Ra ≥ 100,000 (these ranges give 1.59 ≤ Ri ≤ 42.85): (i) the Nusselt number for horizontal orientation with hot surface facing down is less that that of vertical orientation and that of horizontal orientation with hot surface facing up, and (ii) the Nusselt number of vertical orientation is approximately the same as that of horizontal orientation with hot surface facing up. For all surfaces orientations and for the entire ranges of Re and Ra, it was found that increasing the cross flow strength decreases the effective cooling of the surface.     

Numerical study of mixed convection and conduction in a 2-D square ventilated cavity with an inlet at the vertical glazing wall and outlet at the top surface

A steady state numerical study of combined laminar mixed convection and conduction heat transfer in a ventilated square cavity is presented. The air inlet gap is located at the bottom of a vertical glazing wall and air exits the cavity via a gap located at the top surface. Three locations for the opening at the top surface: left (case a), center (case b) and right side (case c) are considered. All the remaining surfaces are considered adiabatic. The mass, momentum and energy conservation equations were solved using the finite volume method for different Rayleigh numbers in the interval of 10⁴ < Ra < 10⁶ and Reynolds number in the interval of 100 < Re < 700. Temperature, flow field, and heat transfer rates are analyzed. The effect of the interaction between ambient conditions outside the glazing and the air inlet gap at the bottom for different air outlet gap positions at the top surface modifies the flow structure and temperature distribution of the air inside the cavity. The Nusselt number as a function of the Reynolds number was determined for the three cases. It was found that configuration for case (a) removes a higher amount of heat entering the cavity compared to cases (b) and (c). This is due to the short distance between the main stream and the glass wall surface. Thus, the forced airflow entering the cavity is assisted by the buoyancy forces, and most of the cavity remains at the inlet flow temperature, which should be appropriate for warm climates. These results may provide useful information about the heat transfer and fluid flow for future studies.     

The developing heat transfer and fluid flow in micro-channel heat sink with viscous heating effect

The numerical modeling of the conjugate heat transfer and fluid flow through the micro-heat sink was presented in the paper, considering the viscous dissipation effect. Three different fluids with temperature dependent fluid viscosity are considered: water, dielectric fluid HFE-7600 and isopropanol. The square shape of the cross-section is considered with D _h  = 50 μm with a channel length L = 50 mm. As most of the reported researches dealt with fully developed fluid flow and constant fluid properties in this paper the thermal and hydro-dynamic developing laminar fluid flow is analyzed. Two different heat transfer conditions are considered: heating and cooling at various Br. The influence of the viscous heating on local Nu and Po is analyzed. It was shown that for a given geometry the local Po and Nu numbers are strongly affected by the viscous heating. Moreover the Po number attains the fully developed value as the external heating is equal with the internal viscous heating.     

Heat transfer during heat sterilization and cooling processes of canned products

In this paper, an analysis of transient heat transfer during heat sterilization and cooling processes of a cylindrical canned product is presented. In the analysis, most practical case including the boundary condition of third kind (i.e., convection boundary condition, leading to 0.1 ≤ Bi ≤ 100) was employed. A simple analytical model for determining effective heat transfer coefficients for such products is developed. For the heat sterilization process, heating coefficient is incorporated into heat transfer coefficient model. An experimental study was performed to measure the thermal center temperatures of the short-cylindrical canned products (i.e., Tuna fish) during heat sterilization at the retort medium temperatures of 115^∘C and 121^∘C, and during cooling process at 16^∘C. The effective heat transfer coefficient model used the experimental temperature data. Using these effective heat transfer coefficients the center temperature distributions were calculated and compared with the experimental temperature distributions. Agreement was found considerably high. The results of the present study indicate that the heat-transfer analysis technique and heat-transfer coefficient model are reliable, and can provide accurate results for such problems. Received on 12 November 1997     

Experimental investigations of flow field and heat transfer characteristics due to periodically pulsating impinging air jets

The paper concentrates on increasing convective heat transfer due to periodically pulsating impinging air jets. A maximum enhancement rate of cooling effectiveness up to 20% could be detected at an excitation Strouhal number of Sr = 0.82 when using a high pulsation magnitude. Reductions up to 5% occured at low Strouhal numbers with coincident high pulsation magnitudes as well. The thermal results were completed with phase-locked flow field investigations by means of PIV and surface visualizations using the oil film method.     

Experimental studies on heat and mass transfer performance of a coiled tube absorber for R134a-DMAC based absorption cooling system

Absorber is an important component in vapor absorption refrigeration system and its performance has greater influence in overall efficiency of absorption machines. Falling film heat and mass transfer in an absorber is greatly influenced by fluid properties, geometry of heat exchanger and its operating parameters. This paper presents on the results of experimental studies on the heat and mass transfer characteristics of a coiled tube falling film absorber, using 1,1,1,2-Tetrafluroethane(R-134a) and N-N Dimethyl Acetamide (DMAC) as working fluids. The effects of film Reynolds number, inlet solution temperature and cooling water temperature on absorber heat load, over all heat transfer coefficient and mass of refrigerant absorbed are presented and discussed. Normalized solution and coolant temperature profiles and refrigerant mass absorbed along the height of absorber are also observed from the experimental results. The optimum over all heat transfer coefficient for R-134a–DMAC solution found to be 726 W/m²K for a film Reynolds number of 350. The R-134a vapour absorption rate is maximum in the normalized coil height of 0.6 to 1.     

Experimental investigation on flow and heat transfer performance of a novel heat fin-plate radiator for electronic cooling

Within the electronics industry, high degree of integration and enhanced performance has led to high heat dissipation electronic devices. This has identified the future development of very high heat flux components. In this paper, a novel and high efficient diffusion welded heat fin-plate radiator (HFPR) was proposed and designed. Various parameters affect the thermal performance of HFPR. The effect of three parameters: the working fluid filling ratios (8% < FR < 70%), the vacuum degrees (0.001 Pa < VD < 0.1 Pa), and the air flow velocities (0.5 m/s < u < 6 m/s) were investigated experimentally. Using distilled water and ethanol as working fluids, a series of tests were carried out to find the influence of the above parameters on steady-state heat transfer characteristics of HFPR. The experimental results indicated that the filling ratio and vacuum degree had a significant influence on thermal performance of HFPR. Also compared with cooling performance using distilled water and ethanol, the HFPR cooling component using distilled water had a stronger heat dissipation capacity for the same filling ratio. The results also can provide a basis for optimal design of HFPR structure.     

Effect of heater size on ultrasonic enhancement of boiling in water and surfactant solutions

Experiments were performed to study enhancement of heat transfer from the wire of d = 50 µm and the tube of d = 1.5 mm in subcooled pool boiling by ultrasonic waves. The working fluids are clean water and Alkyl (8-16) Glucoside surfactant solutions of different concentrations and bulk temperature 30 °C. The wire resistance was translated to the temperature, using the calibration data, the temperature of the tube was measured by thermocouple. The differences between effect of ultrasonic field on boiling in water for heaters of d = 50 µm and d = 1.5 mm may be summarized as follows: for boiling on the wire of d = 50 µm in subcooled water, T_b = 30 °C, enhancement of heat transfer coefficient due to applied ultrasonic field is about 70% and 20% at heat flux q = 620 kW/m² and q = 1350 kW/m², respectively. For boiling in surfactant solutions at the same boiling conditions enhancement of heat transfer coefficient is in the range of 5–10% at heat flux q = 620 kW/m² and 10–16% at heat flux q = 1350 kW/m² depending on solution concentration. For boiling on the tube of d= 1.5 mm in subcooled water, T_b= 30 ℃, enhancement of heat transfer coefficient due to applied ultrasonic field is about 50% and 45% at heat flux q = 500 kW/m² and q = 2500 kW/m², respectively. The same values are obtained for boiling in surfactant solution of concentration C = 250 ppm. For the wire of d = 50 µm the heat transfer enhancement due to acoustic vibrations in surfactant solutions is not as strong as in water. This fact may be considered as evidence of significant role of relationship between jet flow and ultrasonic field.     

Heat transfer and pressure drop of R-134a condensation in a coiled,double tube

An experimental study was carried out to investigate condensation heat transfer and pressure drop characteristics of R-134a in a coiled double tube oriented with its helix axis in the vertical direction. Measurements were obtained at inlet pressure of 815 kPa for refrigerant mass flux ranging from 95 to 710 kg/m²s and cooling water Reynolds number varying from 1000 to 14000. Presented results illustrate the effects of refrigerant mass flux and average condensation temperature difference on the condensation heat transfer coefficient and pressure drop. Comparison with relevant data from other sources indicates a reasonable agreement. An empirical correlation was obtained for predicting condensation heat transfer coefficient. The present study may be considered of a practical and theoretical interest for the design of the helical double-tube condensers using R-134a as the working fluid. M. El-Sayed Mosaad is on leave from Mechanical Engineering Department, Mansoura University, Egypt.     

Effects of injection angle orientation on concave and convex surfaces film cooling

The present study employs a transient liquid crystal thermography to measure film cooling performance over constant curvature of concave and convex surfaces. This work investigates detailed distributions of both film cooling effectiveness and heat transfer coefficient on concave and convex surfaces with one row of injection holes inclined stream-wise at 35° at four blowing ratios (0.5, 1.0, 1.5 and 2.0) on four test pieces with different hole configurations. All test models have a row of discrete holes with a stream-wise injection angle (γ of 35° and a pitch-to-diameter ratio (P/d) of 3. The current work examines four different injection configurations, one with simple and three with 8° forward-expanded holes. Three compound angles of 0, 45 and 90° with air (ρ_c/ρ_m = 0.98) as coolants are tested under the mainstream Reynolds number (Re_d) of 2300 on concave surface, and 1700 on convex surface. Measured results of the concave surface show that both the span-wise averaged heat transfer coefficient and film cooling effectiveness increase with blowing ratios for all tested models. Higher heat transfer levels induced by large flow disturbance of compound-angle injection also lead to poorer overall film cooling performance, especially at high blowing ratio and large span-wise injection angle. Present results show that the best surface protection on the concave surface over the widest range of M can be provided by the forward-expanded holes with β = 0° (Model-B), followed by the forward-expanded holes with β = 45° (Model-C). Convex surface results show that the compound-angle injection indicates increases in both film cooling effectiveness and heat transfer at moderate and high blowing ratios. The forward-expanded hole with simple-angle injection provides the best film performance because of high film cooling effectiveness and low heat transfer coefficient at blowing ratio of 0.5.     

Numerical simulation of fluid flow and heat transfer characteristics in channel with V corrugated plates

A detailed numerical study is carried out to investigate fluid flow and heat transfer characteristics in a channel with heated V corrugated upper and lower plates. The parameters studied include the Reynolds number (Re = 2,000–5,500), angles of V corrugated plates (θ = 20°, 40°, 60°), and constant heat fluxs (q″ = 580, 830, 1,090 W/m²). Numerical results have been validated using the experimented data reported by Naphon, and a good agreement has been found. The angles of V corrugated plates (θ) and the Reynolds number are demonstrated to significantly affect the fluid flow and the heat transfer rate. Increasing the angles of V corrugated plates can make the heat transfer performance become better. The increasing Reynolds number leads to a more complex fluid flow and heat transfer rate. The numerical calculations with a non-equilibrium wall function have a better accuracy than with a standard wall function for solving high Reynolds numbers or complex flow problems.     

Accurate heat transfer measurements using thermochromic liquid crystal. Part 2: Application to a rotating disc

Encapsulated thermochromic liquid crystal (TLC) can accurately measure surface temperature in a variety of heat transfer and fluid-flow experiments. In Part 1 of this two-part paper, two narrow-band liquid crystals were specifically calibrated for application to experiments on a disc rotating at high speed (5000 rpm). Part 2 describes how these crystals were used to measure the surface temperature on the disc in a transient experiment that models the flow of internal cooling air in a gas turbine. The TLC was viewed through the transparent polycarbonate disc using a digital video camera and strobe light synchronised to the disc frequency. The convective heat transfer coefficient, h, was subsequently calculated from the one-dimensional solution of Fourier’s conduction equation for a semi-infinite wall. The analysis accounted for the exponential rise in the air temperature driving the heat transfer, and for experimental uncertainties in the measured values of h. The paper focuses on the method used, and sample experimental results are provided to demonstrate the accuracy and potency of the technique.     

Coupled heat and mass transfer in mixed convection over a vertical flat plate embedded in saturated porous media: PST/PSC or PHF/PMF

A boundary layer analysis is used to investigate the heat and mass transfer characteristics of mixed convection about a vertical flat plate embedded in a saturated porous medium under the coupled effects of thermal and mass diffusion. The plate is maintained at prescribed surface temperature/concentration (PST/PSC) or prescribed heat/mass flux (PHF/PMF). The nonsimilar governing equations are obtained by using a suitable transformation and solved by Keller box method. Numerical results for the local heat transfer rate and the local mass transfer rate are presented for various parameters. The local heat and mass transfer rates increase with increasing n and m and buoyancy parameter ξ. When buoyancy parameter ξ is very small (large) the value of local Nusselt and the local Sherwood number correspond with the pure forced (free) convection, respectively. Increasing buoyancy ratio N (or N ^*) increases the local heat and mass transfer rates. It is apparent that Lewis number has a pronounced effect on the local mass transfer rate than it does on the local heat transfer rate. Furthermore, increasing Lewis number decreases (increases) the local heat (mass) transfer rate. Received on 8 December 1997     

Hydromagnetic flow between two porous disks rotating about non-coincident axes

Hydromagnetic flow between two porous disks rotating with same angular velocity Ω about two noncoincident axes has been studied in the presence of a uniform transverse magnetic field. An exact solution of the governing equations has been obtained in a closed form. It is found that the primary velocity f/Ωl increases and the secondary velocity g/Ωl decreases with increase in either Reynolds number Re or the Hartman number M. It is also found that the torque at the disk η= 0 increases with increase in either M^2 or K^2. On the other hand there is no torque at the disk η= 1 for large M^2 and K^2. The heat transfer characteristic has also been studied on taking viscous and Joule dissipation into account. It is seen that the temperature increases with increase in either M^2 or K^2. It is found that the rate of heat transfer at the disk η= 0 increases with increase in either M or K. On the other hand the rate of heat transfer at the disk η= 1 increases with increase in K but decreases with increase in M.     

Hybrid turbulence simulation of spray impingement cooling: The effect of vortex motion on turbulent heat flux

Spray impingement has been a major interest of researchers in the areas of spray cooling, internal combustion, fire suppression and spray cooling, etc. for a long time. Numerous studies have been done in the area of spray cooling. Spray cooling with phase change takes advantage of relatively large amounts of latent heat and is capable of removing high heat fluxes from the surface, which has generated the interest of many researchers. In this paper, the turbulent characteristics of vapor formed during the spray impingement are studied. Water and gasoline are used in the numerical analysis of the two‐phase spray impingement on a heated wall. Hybrid turbulence modeling was used for the analysis where the subgrid scale model was employed away from the wall and k–ε model was used near the wall. Gasoline, at 298 K, was sprayed on the heated wall, kept constant at 650 K. The surrounding temperature was maintained at 400 K at the start of the simulation. In case of water and gasoline at Reynolds number 2750, the heated wall was kept constant at 400 K and the surrounding temperature was maintained at 298 K at the start of the simulations. The nozzle diameter of 100µm was used for this study, with the nozzle plate spacing ratio at 10. The spray was impinged on the flat plate at angles of 0, 15, and 30^°. Root mean‐squared velocities and turbulent heat flux were plotted in the water spray impingement for the different angles of impingement. The effect of turbulence on the heat transfer was observed. The effect of vortex motion on the turbulent heat flux values was analyzed using different Reynolds numbers of impingement and at different angles in case of gasoline. The turbulent heat flux attained the maximum values with high vortex formation. Upwash of fluid transported heat away from the wall, producing higher heat flux values in the region. Copyright © 2008 John Wiley & Sons, Ltd.