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.     

Heat transfer due to a round jet impinging normal to a circular cylinder

An experimental investigations of heat transfer for a stationary isothermal circular cylinder exposed normal to an impinging round air-jet has been reported. The circumferential heat transfer distributions as well as axial Nusselt number is measured. The measurements are taken as a function of the Reynolds number ranging from 3.8 × 10³ to 4 × 10⁴, the cylinder separation distance to the nozzle diameter (z/d) varying from 7 to 30, and the nozzle to cylinder diameter ratio (d/D) changing from 0.06 to 0.14. The output results indicated that the axial and radial distributions of the local heat transfer peaked at the impingement point. The heat transfer rate increases as the values of z decreases, for the same d and Re. The drop-off of the Nusselt number with increasing axial distance or radial angle from the impingement point was more pronounced for smaller z and d. The peripheral and surface average Nusselt numbers were determined by integration. The experimental data was used to produce correlations for both average and stagnation point heat transfer. Received on 4 January 1999     

Unsteady flow with heat and mass transfer due to impingement of slot jet on an inclined plate

The unsteady laminar incompressible boundary layer flow due to a two-dimensional slot jet on a flat plate at an angle of attack has been studied. The unsteadiness in the flow field is due to the free stream velocity distribution or wall temperature (concentration) which varies with time. The governing partial differential equations in primitive variables have been solved numerically using an implicit finite-difference scheme in combination with the quasilinearization technique. The effect of the variation of the free stream velocity distribution with time is found to be more pronounced on the skin friction than on the heat or mass transfer. The Prandtl number and the variation of the wall temperature with time strongly affect the heat transfer. Similarly, the Schmidt number and the variation of the concentration at the wall with time strongly affect the mass transfer. Beyond a certain critical value of the viscous dissipation parameter, the plate gets heated instead of being cooled.     

Boiling heat transfer characteristics of nanofluids jet impingement on a plate surface

The jet boiling heat transfer of a bar water–CuO particle suspensions (nanofluids) jet impingement on a large flat surface was experimentally investigated. The experimental results were compared with those from water. The quantificational effects of the nanoparticles concentration and the flow conditions on the nucleate boiling heat transfer and the critical heat flux (CHF) were investigated. The experimental data showed that the jet boiling heat transfer for the water–CuO nanofluid is significantly different from those for water. The nanofluids have poor nucleate boiling heat transfer compared with the base fluid due to that a very thin nanoparticle sorption layer was formed on the heated surface. The CHF for the nanofluid increased compared with that of water. The reasons were that the solid–liquid contact angle decreased due to a very thin sorption layer on the heated surface and the jet and agitating effect of the nanoparticles on the subfilm layer enhance supply of liquid to the surface.     

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.     

Numerical analysis of turbulent convection heat transfer from an array of perforated fins

Numerical investigation is made for three-dimensional fluid flow and convective heat transfer from an array of solid and perforated fins that are mounted on a flat plate. Incompressible air as working fluid is modeled using Navier–Stokes equations and RNG based k ? ? turbulent model is used to predict turbulent flow parameters. Temperature field inside the fins is obtained by solving Fourier’s conduction equation. The conjugate differential equations for both solid and gas phase are solved simultaneously by finite volume procedure using SIMPLE algorithm. Perforations such as small channels with square cross section are arranged streamwise along the fin’s length and their numbers varied from 1 to 3. Flow and heat transfer characteristics are presented for Reynolds numbers from 2 × 10⁴ to 4 × 10⁴ based on the fin length and Prandtl number is taken Pr = 0.71. Numerical computations are validated with experimental studies of the previous investigators and good agreements were observed. Results show that fins with longitudinal pores, have remarkable heat transfer enhancement in addition to the considerable reduction in weight by comparison with solid fins.     

Experimental and numerical heat transfer investigation of an impingement jet array with V-ribs on the target plate and on the impingement plate

The secondary vortex structure of an impingement jet system is enhanced by V-ribs on both the impingement and target plates. Numerical and experimental investigations are conducted to study the flow field and heat transfer resulting from V-rib turbulators in an impingement cooling configuration. Three different cases are tested: V-ribs on both the impingement and target plates (V-rib), V-ribs only on the impingement plate (V-rib-impingement) and V-ribs only on the target plate (V-rib-target). The experiment is carried out on a 9 by 9 inline impingement array test facility. For the transient measurements, narrow band thermochromic liquid crystals (TLC) and thermocouples are applied to obtain the local heat transfer distribution. Pressure taps are used to measure the pressure loss. The numerical simulation is carried out with ANSYS CFX 14, using a steady state Reynolds-Averaged Navier-Stokes (RANS) approach and the Shear Stress Transport (SST) turbulence model. All studies are done for a Reynolds number range of 15,000 to 35,000. There is a good overall agreement between the experimental and numerical results for the cases studied. The detailed flow field from the numerical simulation is used to understand and complement the phenomena observed in the experiment. The evaluation of the flow field confirms that the V-ribs enhance the secondary flow structure in the impingement system and induce a positive heat flux ratio compared to the baseline case. Both experimental and numerical results show a Nusselt number increase for the V-rib-impingement and V-rib configuration, with a highest Nusselt number ratio of 1.16. Notice that the experiment cannot take the rib part into account due to the invalid 1D semi-infinite wall assumption there, while the CFD simulation allows for the consideration of heat transfer on the rib surface and thus complements the heat flux data on the target plate. Depending on the configuration, the CFD simulation shows a heat flux ratio of 1.06–1.34. The pressure loss of the system is comparable to the case with a smooth impingement plate and a smooth target plate.     

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 an annular turbulent impinging jet with a confined wall measured by thermosensitive liquid crystal

The present paper describes the heat transfer characteristics of an annular turbulent impinging jet with a confined wall. The local temperature distribution on the impingement surface was measured using a thermosensitive liquid crystal sheet and an image processor. The net heat flux was evaluated by considering the heat conduction in the heated substrate and the thermal radiation between an upper confining insulated wall and an impingement surface. Distributions of the temperature and Nusselt number on the impingement surface were captured in two-dimensional maps. Effects of the diameter ratio of the annular nozzle, the space between nozzle and impingement surface and the Reynolds number on radial distributions of the local Nusselt number were examined. Experimental formulas of the local Nusselt number were obtained in power-law expressions of r/r_p for the major and minor flow regions.     

Heat transfer characteristics of premixed butane/air flame jet impinging on an inclined flat surface

 A series of experiments were carried out to determine the heat transfer characteristics of a round, premixed butane/air flame jet impinging upwards on an inclined flat plate, at different angles of incidence. The flame was fixed with an equivalence ratio of 1.0, a Reynolds number of 2500 and a plate-to-nozzle distance of 5d, while the inclination angles chosen for investigation were 57°, 67°, 80° and 90°. It was found that the location of the maximum heat flux point would be shifted away from the geometrical impingement point by reducing the angle of incidence. Decreasing the angle of incidence also enhanced the maximum local heat flux, while reduced the average heat transfer. The present study presented the effect of angle of incidence on the heat transfer characteristics of an impinging butane/air flame jet, which had been rarely reported in previous similar studies. Received on 11 October 2000 The authors wish to thank The Hong Kong Polytechnic University for the financial support of the present study.     

Heat transfer and pressure distributions of an impinging jet on a flat surface

Experiments were conducted to determine the heat transfer and surface pressure characteristics of a round jet impinging normal on isothermal flat plate. Three nozzles of exit diameters 3, 5 and 7?mm have been used. The local heat transfer rates have been estimated from the outputs of three-wire differential thermocouple heat flux sensors. The results cover a Reynolds number range of 3400 to 41?000 and dimensionless separation distances varies from 6 to 58. The static pressure distributions along the impingement surface are found to be similar and closer to the heat transfer variations at the same configurations. A simple correlation is derived for the average heat transfer coefficients within ±10% deviation from the output data covering the complete range of experimental limits. The predicted values of Nusselt number have also been compared with the results obtained from the literature. The agreement was generally good.     

Vaporization of single liquid drops in an immiscible liquid part II: Heat transfer characteristics

Heat transfer characteristics during the vaporization process of a pentane or furan drop in an aqueous glycerol of high viscosity has been studied. With the progress of vaporization, the overall heat transfer coefficient related to the liquid-liquid interfacial area of a two-phase bubble increases monotonically, and influences of initial drop diameter and temperature difference reduce. Some convection or circulation seems to occur in the unvaporized-liquid phase.

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Verdampfung einzelner Flüssigkeitstropfen in einer nicht mischbaren Flüssigkeit. Teil II: Der Wärmeübergang

Zusammenfassung In dieser Arbeit wird der Wärmeübergang während der Verdampfung von Pentan- und Furan-Tropfen in einer wässerigen Glyzerinlösung hoher Viskosität untersucht. Mit fortschreitender Verdampfung steigt der Wärmeübergangskoeffizient, bezogen auf die Grenzfläche flüssig-flüssig der zweiphasigen Blase monoton an, wobei Einflüsse des anfänglichen Tropfendurchmessers und der Temperaturdifferenz abnehmen. In der nichtverdampften Flüssigkeitsphase scheint Konvektion oder Zirkulation aufzutreten.

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Nomenclature A total surface area of two-phase bubble - A_L liquid-liquid interfacial area of two-phase bubble - D equivalent spherical diamter of two-phase bubble - D_i initial drop diameter - h average overall heat transfer coefficient related to A - h_c average outside heat transfer coefficient related to A - q local outside heat transfer coefficient - h_L average overall heat transfer coefficient related to A_L - h_Lc average outside heat transfer coefficient related to A_L - k_c thermal conductivity of continuous-phase liquid - k_dl thermal conductivity of dispersed-phase liquid - k_v correction factor of velocity [cf. Eq.(2)] - Nu_c =h_c D/k - Nu_c =h_c D/k_c - Pe_c =UD/_c - Pr_c =_c/_c - Q cumulative heat transferred into two-phase bubble - q local heat flux - r radial distance in spherical co-ordinates - R radius of two-phase bubble - T temperature - T_L interface temperature between continuousphase and dispersed-phase component in liquid phase - T bulk temperature - T temperature difference - T nominal temperature difference - U velocity of rise of two-phase bubble - u velocity gradient in r direction [cf. Eq.(9)] - u_r velocity component in r direction - u velocity component in direction - V volume of two-phase bubble - V_dl volume of dispersed-phase component in liquid phase - X defined in Eq.(7) - Y defined in Eq.(8) - Z defined in Eq.(12) - _c thermal diffusivity of continuous-phase liquid - half opening angle of vapor phase in two-phase bubble - average thickness of dispersed-phase component in liquid phase [cf. Eq.(22)] - angle in spherical co-ordinates - vaporization ratio - time     

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.     

Effects of temperature-dependent fluid properties on heat transfer due to an axisymmetric impinging gas jet normal to a flat surface

Heat transfer due to an axisymmetric laminar gas jet impinging onto a flat solid surface of uniform temperature is studied numerically, taking into account the temperature dependence of all fluid physical properties. Numerical solutions are obtained for the jet Reynolds numbers 200–2000, jet mouth-to-surface distances 1–4 times the jet nozzle diameter, and for helium-4, air, and carbon dioxide. Effects of the temperature dependence of the fluid properties are investigated using various kinds of reference temperatures and a viscosity correction method. A method of estimating the values of the local Nusselt number for temperature-dependent fluid from the constant-property solutions is proposed.Die Wärmeübertragung durch einen auf eine flache Oberfläche gleichförmiger Temperatur achsensymmetrisch auftreffenden Gasstrahl wird numerisch unter Berücksichtigung der Temperaturabhängigkeit der physikalischen Eigenschaften von Fluiden untersucht. Die numerischen Lösungen werden für die Reynoldschen Strahlzahlen von 200 bis 2000, für die Abstände vom Düsenmund zur Oberfläche vom 1- bis 4-fachen des Düsenstrahldurchmessers und für Helium-4, Luft und Kohlendioxyd erhalten. Die Wirkungen der Temperaturabhängigkeiten von Fluideigenschaften werden unter Verwendung verschiedener Bezugstemperaturen und einer Viskositätskorrekturmethode untersucht. Ausgehend von der Lösung für konstante Stoffwerte wird eine Methode zur Schätzung der Werte der lokalen Nusseltzahl für temperaturabhängige Fluide vorgeschlagen.     

Impingement heat transfer at a circular cylinder due to a submerged slot jet of water

An experimental investigation was carried out on the heat transfer due to a submerged slot jet of water impinging on a circular cylinder in crossflow. The cylinder diameter and the slot width are of the same order of magnitude, specifically D_s = 2.0 and 3.0 mm and D_c = 2.5 and 3.0 mm. The experimental apparatus allowed variation of the slot width, the cylinder diameter, and the distance from nozxle exit to heater. Conditions of impingement from the bottom (ascending flow) were taken into consideration as well as impingement from above (descending flow). The Nusselt number was determined as a function of Reynolds and Prandtl numbers in the range 1.5 × 10³ < Re < 2.0 × 10⁴, 2.7 < Pr < 7.0, and 1.5 ≤ z/D_s ≤ 10. The experimental data were correlated with a simple equation that fits 90% of the data with a precision of 20%.     

Heat transfer investigation of an array of jets impinging on a target plate with detached ribs

This combined experimental and numerical study focuses on impingement jet cooling in combination with detached rib turbulators on a flat target for turbomachinery applications. The investigated impingement array consists of an impingement plate with 9 × 9 jet holes with diameter D and a target plate with detached ribs installed beneath the jet hole. The effects of different separation distances (H/D=3-5), jet Reynolds numbers (15,000-35,000) and rib clearances (0.3D and 0.08D) are investigated. The heat transfer is investigated experimentally by the transient liquid crystal (TLC) method. A computational fluid dynamics (CFD) model is carried out within the software package ANSYS CFX. This model uses a steady-state three-dimensional Reynolds-Averaged Navier-Stokes (RANS) approach with the Shear Stress Transport (SST) turbulence model. Numerical simulations allow detailed insight into the fluid mechanics of the complex flow field and complement experimental measurements. Detached ribs in the impingement channel have a strong influence on the flow field and can increase the global Nusselt number by up to 4% if the ribs have adiabatic boundary conditions. The usage of the detached rib reduces the relative discharge coefficient by up to 11% compared to a smooth target.     

Heat transfer to an underlying fluid due to the axisymmetric spreading of material on the surface

Similarity boundary layer solutions are found for the fluid underlying an axisymmetric spreading material layer. Three thermal boundary conditions for the fluid-material interface are considered, corresponding to constant temperature interface, adiabatic interface with heat source at origin, and constant heat flux interface. The boundary layer thickness is proportional to the distance from origin. Physical significance is discussed.     

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.