Single-phase convection and boiling heat transfer: Confined single and array-circular impinging jets

In the present study, the effects of diverse situations of confinement on heat transfer from single and array-circular jet impingements are carefully investigated over various heat transfer regimes of single-phase convection and fully developed nucleate boiling. For the single, circular, unconfined free-surface jet, the transition to turbulence was observed to start around x/d = 5.5 and end around x/d = 9. For the array-circular jet, however, the wall jet structure yielded no transition to turbulence for all the tested cases, instead monotonically decreasing the convection coefficient. Conversely, the single-circular jet experienced the transition for V ? 6.1 m/s. For the confined submerged jet, the transition length was very short due to the vigorous mixing driven by lateral velocity components, and the locus of the secondary peak moved downstream as velocity increased. The temperature distributions of the confined array-circular jet were fairly uniform over the whole heated surface. The averaged single-phase convection coefficients indicated that the confined jet provided the most uniform convection in the lateral direction.     

Heat transfer from an obliquely impinging circular, air jet to a flat plate

A series of experiments was conducted for the measurement of local convective heat transfer coefficients for an obliquely impinging circular air jet to a flat plate. In the experiments, the oblique angles selected were 90°, 75°, 60° and 45°, with 90° being a vertical jet. Two different Reynolds numbers of 10,000 and 23,000 were considered for the purpose of comparison with previous data available in the literature. Another parameter varied in the measurements was the dimensionless jet-to-plate distance, L/D. Four values of L/D(2, 4, 7, and 10) were considered in the experiments. The experiments were conducted using the preheated wall transient liquid-crystal technique. Liquid-crystal color changes were recorded with a video system. Local convective heat transfer coefficients were obtained through the surface transient temperatures that were related to the recorded color information. Detailed local heat transfer coefficients were presented and discussed in relation to the asymmetric wall jet upon impingement of the jet flow. Results of experiments show that, for a given flow situation, the point of maximum heat transfer shifts away from the geometrical impingement point toward the compression side of the wall jet on the axis of symmetry. The shift is more pronounced with a smaller oblique angle (larger jet inclination) and a smaller jet-to-plate distance. Comparisons of experimental results with existing heat transfer data for both obliquely impinging jets and vertical impinging jets are made. The effect of oblique angles on heat transfer was assessed.     

Heat transfer characteristics of a planar water jet impinging normally or obliquely on a flat surface at relatively low Reynolds numbers

The heat transfer characteristics of a planar free water jet normally or obliquely impinging onto a flat substrate were investigated experimentally. The planar jet issued from a rectangular slot nozzle with a cross section of 1.62 mm × 40 mm. The mean velocity at the nozzle exit ranged from 1.5 to 6.1 m s⁻¹. The corresponding Reynolds number range based on the nozzle gap and the mean velocity was 2200–8800. Constant heat-flux conditions were employed at the solid surface. Various impingement angles between the vertical planar jet and the inclined solid surface were investigated: 90° (normal collision), 70°, 60°, and 50°. In the case of normal collisions, the Nusselt number is high at the impingement line, and decreases with departures from it. The stagnation Nusselt numbers were compared to the predictions of several correlations proposed by other researchers. In oblique collisions, the profiles of the local Nusselt numbers are asymmetric. The locations of the peak Nusselt numbers do not coincide with the geometric center of the planar jet on the surface.     

Heat transfer from a flat surface to an inclined impinging jet

An experimental study was carried out to investigate the effect of the inclination jet on convection heat transfer to horizontal flat plate. Local heat transfer determined as a function is of three parameters including inclination angle of the air jet relative to the plate in range of 90° ≤ θ ≤ 45°, jet-to-plate spacing in range of 2 ≤ L/D ≤ 8 and Reynolds number in range of 1,500 ≤ Re ≤ 30,000. The results show that the maximum heat transfer point moves towards the uphill side of the plate and the maximum heat transfer decreases as the inclination angle decreases. The correlations were conducted to predict maximum and local Nusselt number as a function of Re, θ, L/D, and x/D for a specific three regions.     

Surface condition effects on flame impingement heat transfer

Flame impingement heating is used in many industrial applications, including the heating and melting of both glass and metal. This heating process usually comprises multiple heat transfer mechanisms, such as forced convection, thermal radiation, and thermochemical heat release. However, little experimental data are available that can be used to determine the importance of each mechanism. This information would be useful for optimizing the heating process and for developing computer models. The objective of this study was to determine the relative importance of thermal radiation and to determine how the thermochemical heat release is affected by the surface properties of the target.This study investigated the heat transfer from oxygen-enhanced, natural gas flames (15 kW) impinging normal to a water-cooled metal disk (d_b = 135 mm) segmented into concentric calorimetric rings. Polished, untreated, and blackened surfaces were used to study emissivity effects. The heat flux to the blackened and polished surfaces was the highest and lowest, respectively. The flux to untreated surfaces was between the highest and lowest fluxes. The largest difference in the flux, between the polished and blackened surfaces, was only 9.8%. Catalyticity effects were investigated by using alumina-coated (nearly noncatalytic), untreated, and platinum-coated (highly catalytic) surfaces. The heat flux to platinum-coated surfaces was the highest. The fluxes to untreated surfaces were similar to those for alumina-coated surfaces. The largest difference in the flux, between the platinum-coated and the alumina-coated surfaces, was only 12%. Therefore, both nonluminous flame radiation and the thermochemical heat release from surface catalytic reactions were relatively small fractions of the total heat flux.     

Heat transfer in an oscillating vertical annular liquid column open to atmosphere

The heat transfer from a surface heated with constant heat flux to an oscillating vertical annular liquid column having an interface with the atmosphere is investigated experimentally in the present paper. The analysis is carried out for the case of different oscillation frequencies while the displacement amplitude remains constant. Based on the experimental data a correlation equation is obtained for the cycle-averaged Nusselt number as a function of kinetic Reynolds number.     

Heat transfer in an unevenly heated porous layer

For a uniform saturated porous layer heated from below, the dependence of the quantity of heat transferred on the distribution of the heat source is investigated. It is found, using perturbation methods and numerical techniques, that very small nonuniformities in the heat source having the same wavelength as the preferred convection mode significantly reinforce natural convection.     

Heat transfer from an open-wedge cavity to a symmetrically impinging slot air jet

Heat transfer from an open-wedge cavity to a symmetrically impinging slot air jet is investigated at the present study. The effect of the cavity angle was mainly examined on the Nusselt number distribution. Based on the results, heat transfer was generally poor at the vicinity of the apex, rising to form a maximum at the impingement and then followed by a moderate decline at further distances. The region of maximum heat transfer on the surfaces shifted outward the cavity as the cavity angle was decreased. Also, average Nusselt number over an effective length of the surface remained almost constant and independent of the cavity angle for a specified jet Reynolds number and nozzle-to-apex spacing.     

Effect of variable duty cycle flow pulsations on heat transfer enhancement for an impinging air jet

A series of experiments has been conducted in which a pulsed air jet is impinged upon a heated surface for the purpose of enhancing heat transfer relative to the corresponding steady air jet. Traditional variables such as jet to plate spacing, Reynolds number, and pulse frequency have been investigated. One additional flow variable – the duty cycle – representing the ratio of pulse cycle on-time to total cycle time is introduced and shown to be significant in determining the level of heat transfer enhancement. Specifically, heat transfer enhancement exceeding 50% is shown for a variety of operating conditions. In each case, the duty cycle producing the best heat transfer is shown to depend upon each of the other flow parameters. Recommendations are made for further experimentation into optimizing the duty cycle parameter for any particular application.     

Heat transfer to non-Newtonian flows over a cylinder in cross flow

Over a range of 102<Re^*<5800, 6.5<Pr^*<79, and 0.6<n<1, circumferential wall temperatures for water and aqueous polymer (purely viscous) solution flows over a smooth cylinder were measured experimentally. The cylinder was heated by passing direct electric current through it. Aqueous solutions of Carbopol 934 and EZ1 were used as power-law non-Newtonian fluids. The peripherally averaged heat transfer coefficient for purely viscous non-Newtonian fluids, at any fixed flow rate, decreases with increasing polymer concentration. A new correlation is proposed for predicting the peripherally averaged Nusselt number for power-law fluid flows over a heated cylinder in cross flow.     

Experimental study of heat transfer characteristics due to confined impinging two-dimensional jets

Experimental results are presented for characteristics of impingement heat transfer caused by three slot jets. Experimental values were obtained for the dimensionless distance H = 0.5−3, dimensionless pitch P = 6−16, and Reynolds number Re = 500−8000. For laminar impinging flow, they were compared with numerical results. For turbulent impinging flow, two peaks of the local Nusselt number were obtained behind the second nozzle. The position of the second peak approached the nozzle as the space between nozzle and impinged surface decreased. The average Nusselt number between the central and second nozzles was determined from the ratio P/H and the Reynolds number based on the pitch of the nozzles.     

Heat transfer associated to a hot surface quenched by a jet of oil-in-water emulsion

In hot rolling, the mechanical properties of steel alloys are conditioned by the rolling process but a great part is ensured by the cooling of the hot strip mill. Well controlling this cooling rate and its homogeneity is thus of primary importance for obtaining steels with desired mechanical properties. As the water used in the cooling stage of the rolling process can be polluted by oil (in hot mill strip, some oil is used to lubricate the rolls and a part of it can pollute the water), it is important to know how much varies the cooling rates when water is polluted. In this study, transient cooling has been investigated during quenching of a hot metal disk with various subcooled oil-in-water emulsion jets. The aim of this work is to compare the cooling efficiency of oil-in-water emulsion jet with a pure water jet. Experimental investigations of axisymmetric jet impingements on a preheated hot metal disk (500-600 °C) have been performed with various oil-in-water emulsions. The transient cooling heat fluxes on the quenched side are estimated by coupling the measurement of the temperature field of the other side (rear face) with a semi-analytical inverse heat conduction model.     

Particle size effect on heat transfer performance in an oscillating heat pipe

The effect of Al₂O₃ particles on the heat transfer performance of an oscillating heat pipe (OHP) was investigated experimentally. Water was used as the base fluid for the OHP. Four size particles with average diameters of 50 nm, 80 nm, 2.2 μm, and 20 μm were studied, respectively. Experimental results show that the Al₂O₃ particles added in the OHP significantly affect the heat transfer performance and it depends on the particle size. When the OHP was charged with water and 80 nm Al₂O₃ particles, the OHP can achieve the best heat transfer performance among four particles investigated herein. In addition, it is found that all particles added in the OHP can improve the startup performance of the OHP even with 20 μm Al₂O₃ particles.     

Heat transfer analysis according to condensation flow structures in a minichannel

An experimental investigation of complete condensation flow is undertaken for a range of mass flow rates between 3.4 and 13.8 kg/m² s. The associated flow regimes are visualized using an ombroscopic technique. Two major flows are observed (with or without release of bubbles). A critical value of the mass flow rate is obtained at the transition between these two regimes. The visualization also enables a local parameter to be determined: the void fraction. The influence of the presence of a bubbly zone is highlighted by the heat transfer and pressure drops. Finally, the dependence of the critical value of the mass flow rate on the temperature of the secondary flow is obtained.     

Heat transfer in a vertically-layered porous medium heated from below

Some studies already made have investigated the criterion for onset of convection and heat and mass flow distributions in a porous slab composed of horizontal layers of different materials. This paper reports a study of such criteria for the case where the slab is composed of vertically-aligned strata with different permeabilities and thermal conductivities. This has particular relevance to where blocks of different materials abut in a vertical plane, as well as the case of very narrow highly permeable vertical layers which represent vertical faults in a geological structure. Results indicate that permeability and/or thermal conductivity contrasts between layers can significantly affect the flow pattern and the spatial distribution of the surface heat flux. The concentration of flow in highly permeable faults produces marked irregularities in the heat flow through the surface above them.     

Heat transfer and skin-friction in a turbulent boundary layer under a non-equilibrium longitudinal adverse pressure gradient

The experimental data on the effect of weak and moderate non-equilibrium adverse pressure gradients (APG) on the parameters of dynamic and thermal boundary layers are presented. The Reynolds number based on the momentum thickness at the beginning of the APG region was Re^** = 5500. The APG region was a slot channel with upper wall expansion angles from 0 to 14°. The profiles of the mean and fluctuation velocity components were measured using a single-component hot-wire anemometer. The friction coefficients were determined using two methods, namely, the indirect Clauser method and the direct method of weighting the lower wall region on a single-component strain-gage balance. The heat transfer coefficients were determined by a transient method using an IR camera. It is noticed that in the pressure gradient range realized the universal logarithmic region in the boundary layer profile is conserved. The values of the relative (divided by the parameters in zero gradient flow at the same value of Re^**) friction and heat transfer coefficients, together with the Reynolds analogy factor, are determined as functions of the longitudinal pressure gradient. The values of the relative friction coefficient reduced to c_f/c_f0 = 0.7 and those of the heat transfer to St/St₀ = 0.9. A maximum value of the Reynolds analogy factor (St/St₀)/(c_f/c_f0) = 1.16 was reached for the pressure gradient parameter β = 2.9.     

Heat transfer to air–water annular flow in a horizontal pipe

Two-phase air–water flow and heat transfer in a 25 mm internal diameter horizontal pipe were investigated experimentally. The water superficial velocity varied from 24.2 m/s to 41.5 m/s and the air superficial velocity varied from 0.02 m/s to 0.09 m/s. The aim of the study was to determine the heat transfer coefficient and its connection to flow pattern and liquid film thickness. The flow patterns were visualized using a high speed video camera, and the film thickness was measured by the conductive tomography technique. The heat transfer coefficient was calculated from the temperature measurements using the infrared thermography method. It was found that the heat transfer coefficient at the bottom of the pipe is up to three times higher than that at the top, and becomes more uniform around the pipe for higher air flow-rates. Correlations on local and average Nusselt number were obtained and compared to results reported in the literature. The behavior of local heat transfer coefficient was analyzed and the role of film thickness and flow pattern was clarified.     

Heat transfer to a continuous moving flat surface with variable wall temperature

Transient heat transfer from a continuous moving flat surface with varying wall temperature is studied. Numerical results are presented for the transient temperature profiles and heat transfer rates from the wall for Prandtl numbers varying from 0.01 to 1000. Asymptotic solutions for steady state heat transfer rates for large Prandtl number are also presented.     

Heat transfer due to harmonic variation in the free stream temperatures in a flat plate exposed on both sides

The heat transfer through an infinite flat plate is studied when the temperatures of the two free streams surrounding it are varying harmonically with time and out of phase, with a delay period τ_d. The configuration is a simplified model for the heat transfer through the separating wall in the isochoric counter-current heat exchanger. The results show that apart from the τ_d effect, the perturbation parameters depend mainly on the cavity passing frequency f. At the thick plate solution, the combined passing frequency–delay time influences are significant only when the dimensionless frequency is smaller than 10. Within this range τ_d affects seriously not only the temperature perturbation amplitudes (which determine the thermal stresses) but also the heat fluxes and the accumulated energy ones. When f ≥ 10, the plate behaves as two separate semi-infinite slabs. Heat penetration delays greater than one cavity passing period may be possible.     

Comparison of the heat transfer characteristics of supercritical pressure water to that of subcritical pressure water in vertically-upward tubes

A systematic comparison was made between the forced convection heat transfer characteristics of the supercritical pressure water and that of the subcritical pressure water in vertically-upward tubes. It was found that, severe heat transfer deterioration did not occur in the vertically-upward internally-ribbed tube at supercritical pressures, and the variations in the inside wall temperature with the bulk fluid enthalpy experienced three stages, namely, the continuously increasing stage, the smoothly changing stage and another continuously increasing stage at the supercritical pressures; however, at subcritical pressures, there existed at least four stages for the variation of the inside tube wall temperature, i.e., the continuously increasing stage, the basically unchanging stage, the sharply rising stage and another continuously increasing stage. The heat transfer coefficients in the subcritical two-phase region, in which the heat transfer deterioration did not occur, were much greater than those in the heat transfer enhancement region of supercritical pressure water. In the large specific heat region of supercritical pressure water, the enhanced heat transfer was impaired by increasing the heat flux; however, in the subcritical two-phase region, the higher the heat flux, the greater the heat transfer coefficient would be. It was also found that the heat transfer deterioration of supercritical pressure water was similar in mechanism to the DNB (departure from nucleate boiling) at subcritical pressures.