Experimental Investigation Of Heat Transfer Research Articles

Effect of the droplet size of an emulsion dispersion phase in nucleate boiling and emulsion boiling crisis

This paper presents the results of an experimental investigation of heat transfer in a boiling water/(vacuum oil VO-1C) emulsion at the surface of platinum wire 0.10mm in diameter and in a pipe with an inner diameter of 16mm. The influence of the diameter of a disperse phase droplet on the character of heat transfer in nucleate boiling and emulsion boiling crisis has been investigated.

Heat transfer and pressure drop in rectangular channels with crossing fins (a Review)

Channels with crossing finning find wide use in the cooling paths of high-temperature gas turbine blade systems. At different times, different institutions carried out experimental investigations of heat transfer and pressure drop in channels with coplanar finning of opposite walls for obtaining semiempirical dependences of Nusselt criteria (dimensionless heat-transfer coefficients) and pressure drop coefficients on the operating Reynolds number and relative geometrical parameters (or their complexes). The shape of experimental channels, the conditions of experiments, and the used variables were selected so that they would be most suited for solving particular practical tasks. Therefore, the results obtained in processing the experimental data have large scatter and limited use. This article considers the results from experimental investigations of different authors. In comparing the results, additional calculations were carried out for bringing the mathematical correlations to the form of dependences from the same variables. Generalization of the results is carried out. In the final analysis, universal correlations are obtained for determining the pressure drop coefficients and Nusselt number values for the flow of working medium in channels with coplanar finning.

Experimental investigation of heat transfer in helium-xenon mixtures in triangle channels

Correlations are obtained for resistance and heat transfer of a helium-xenon mixture with a helium mass fraction of 9.1% in a triangle channel for Reynolds numbers from 103 to 2.4 × 104 and thermal fluxes up to 9.3 kW/m2. It is shown that the resistances of the turbulent dependence are in agreement and the heat transfer corresponds to the Tetelbaum dependence.

Experimental investigation of heat transfer of helium-xenon mixtures in cylindrical channels

Correlations are obtained for resistance and heat transfer of a helium-xenon mixture with a helium weight concentration from 5.2 to 10.5% in a cylindrical channel for Reynolds numbers from 2 · 103 to 105 and thermal fluxes up to 4 kW/m2. It is shown that with increasing Reynolds numbers in a long cylindrical channel the heat transfer intensity grows to maximum and then substantially decreases.

Experimental investigation of heat transfer in three phase fluidized bed cooling column

In this study, a three-phase (gas-liquid-solid) fluidized bed was used to study the heat transfer characteristics of the system. The system consisted of low density (290 kg/m3) spherical particles of the diameter of 2 cm in 0.25 m cylindrical column with the countercurrent flow of water and air. The experimental investigation and mathematical modeling of heat transfer between the hot air and the cooling water was carried out. The experiments were conducted for the variety of different fluid flow rates and inlet air temperatures, while the air flow rate was kept constant. Based on the obtained experimental results a new correlation for heat transfer in three-phase fluidized system was proposed. The mean percentage error between the experimental and the correlated values of the jHp obtained was 1.69%. The hydrodynamic parameters of the system were also calculated according to the available literature correlations.

Air-Water Cooling System for Switch-Mode Power Supplies

The results are presented of an experimental investigation of heat transfer in an air-liquid cooling system for a Switch-Mode Power Supply (SMPS) for TV digital power amplifiers. Since these SMPSs are characterized by high power and high compactness, thereby making the standard cooling techniques difficult to be used, a new cooling system is developed, using water and air as the cooling media. The active components (MOSFETs) are cooled with a liquid cold-plate, the passive ones (condensers, transformers, coils) with an air flow, in turn cooled by the cold-plate. By inserting the cooling system in an experimental tool where it is possible to control the cooling water, measurements are made of temperature in the significant points of the SMPS. The electric efficiency is also measured. The evaluation of the thermal performance of this cooling system is useful in order to limit its maximum operational temperature. The efficacy of the cooling system is demonstrated; the trends of efficiency and power dissipation are evidenced.

Experimental investigation of heat transfer through porous media in superfluid helium

An experimental investigation of heat transfer through porous media in superfluid helium has been conducted in the framework of the development of porous electrical insulations for superconducting magnet cables cooled by superfluid helium. Several types of porous media with different characteristics were tested and, in particular, samples with pore size diameters of 0.1μm, 1μm, 2μm, 10μm and 20μm. Temperature and pressure were measured between an insulating inner bath and the cryostat bath, communicating only through the porous medium. The cryostat bath is held constant all along the measurement and, for each sample, the tests are performed for bath temperature from 1.4K to 2.1K with 0.1K increment. Depending on the porous medium average pore size diameter, different flow regimes are observed: for porous media with a pore diameter of 0.1 and 1μm, only the Landau regime is observed whereas for porous media with a pore diameter of 2μm, we observed the Landau regime and the Gorter-Mellink regime. For samples with a pore diameter of 10 and 20μm, measurements only permitted to detect the Gorter-Mellink regime. In the laminar regime, the permeability of the samples was determined and it was found that the permeability is constant for bath temperature above 1.9K whereas it increases as the bath temperature decreases from 1.8K to 1.4K. For samples with a pore size diameter of 10 and 20μm, measurement permits only to observe the turbulent regime and the analysis exhibits a constant average tortuosity for each samples, independently of the bath temperature.

English

This paper presents the results of an experimental investigation of heat transfer to the airflow in the square duct. The main purpose of this study is influence of square ducts on the convective heat transfer mechanism. Under this investigation friction factor and the Nusselt Number were calculated. The reason that square ducts increase the fluid flow turbulence near the wall and also increase the heat transfer area. The results of the heat transfer are characterized in terms of the non-dimensional parameters: Nusselt number (Nu), Reynolds number (Re), and Prandtl number(Pr). The experimental measurements are carried out for Reynolds number range of10,000-50000.The heat transfer, friction factor and the pressure drop characteristics of turbulent flow of air (10,000 < Re < 50,000) through square ducts have been studied experimentally. The study shows significant improvements can be achieved with the square duct particularly in high prandtl number.

Experimental investigation of heat transfer and pressure drop characteristics of ammonia–water in a mini-channel annulus

The ammonia–water mixture is commonly used in absorption and compression–resorption heat pumps. The heat transfer performance of a vertically oriented mini-channel annulus operated with an ammonia–water mixture under absorption conditions has been experimentally investigated. Heat exchangers comprised of annuli can be used in compression–resorption heat pumps. Measurements have been executed in a channel with a hydraulic diameter of 0.4mm and a length of 0.8m with an average eccentricity of 0.6. The experiments are used to determine the heat transfer coefficient and pressure drop during absorption for different operating conditions along the channel. The measured heat transfer coefficients vary from 1000 to 10,000Wm−2K−1. Results are presented as function of heat flux, mass flux and vapor quality in order to investigate the dependency of heat transfer coefficients on the given variables. Mass flux is directly measured; vapor quality is obtained from equations of state with pressure and temperature at the inlet and outlet of each channel as input, assuming equilibrium conditions. The heat transfer coefficient increases with increasing mass flux, increasing inlet vapor quality and increasing heat flux. The heat transfer coefficient increases sharply between mass fluxes of 120 and 175kgm−2s−1 at low inlet vapor qualities and constant heat flux. The pressure drop shows an increasing trend with increasing mass flux and vapor inlet quality. The pressure drop measurements have been compared against empirical models from literature originally designed for tubes. One of these models is able to predict the measured pressure drop in the current channel within 25% deviation. The heat transfer performance was compared against empirical models from literature, which show very little agreement with the results from the experiments. The models are intended to predict condensation heat transfer in tubes, so they cannot fully take the annular geometry, eccentricity and mass transfer resistances into account, causing large discrepancies between predicted and experimental heat transfer coefficients.

Experimental Investigation of Heat Transfer and Pressure Drop Characteristics of H-type Finned Tube Banks

H-type finned tube heat exchanger elements maintain a high capacity for heat transfer, possess superior self-cleaning properties and retain the ability to effect flue gas waste heat recovery in boiler renovations. In this paper, the heat transfer and pressure drop characteristics of H-type finned tube banks are studied via an experimental open high-temperature wind tunnel system. The effects of fin width, fin height, fin pitch and air velocity on fin efficiency, convective heat transfer coefficient, integrated heat transfer capacity and pressure drop are examined. The results indicate that as air velocity, fin height and fin width increase, fin efficiency decreases. Convective heat transfer coefficient is proportional to fin pitch, but inversely proportional to fin height and fin width. Integrated heat transfer capacity is related to fin efficiency, convective heat transfer coefficient and finned ratio. Pressure drop increases with the increase of fin height and fin width. Finally, predictive correlations of fin efficiency, Nusselt number and Euler Number are developed based on the experimental data.

Experimental investigation of heat transfer and flow using V and broken V ribs within gas turbine blade cooling passage

Gas turbines are extensively used for aircraft propulsion, land-based power generation, and various industrial applications. With an increase in turbine rotor inlet temperatures, developments in innovative gas turbine cooling technology enhance the efficiency and power output; these advancements of turbine cooling have allowed engine designs to exceed normal material temperature limits. For internal cooling design, techniques for heat extraction from the surfaces exposed to hot stream of gas are based on an increase in the heat transfer areas and on the promotion of turbulence of the cooling flow. In this study, an improvement in performance is obtained by casting repeated continuous V- and broken V-shaped ribs on one side of the two pass square channels into the core of the blade. A detailed experimental investigation is done for two pass square channels with a 180° turn. Detailed heat transfer distribution occurring in the ribbed passage is reported for a steady state experiment. Four different combinations of 60° V- and broken 60° V-ribs in a channel are considered. A series of thermocouples are used to obtain the temperature on the channel surface and local heat transfer coefficients are obtained for Reynolds numbers 16,000, 56,000 and 85,000 within the turbulent flow regime. Area averaged data are calculated in order to compare the overall performance of the tested ribbed surface and to evaluate the degree of heat transfer enhancement induced by the rib. Flow within the channels is characterized by heat transfer enhancing ribs, bends, rotation and buoyancy effects. A series of experimental measurements is performed to predict the overall performance of the channel. This paper presents an attempt to collect information about the Nusselt number, the pressure drop and the overall performance of the eight different ribbed ducts at the specified Reynolds number. The main contribution of this study is to evaluate the best combination of rib arrangements throughout the two pass cooling channels. After a series of experiments, it can be concluded that the distribution of peaks in heat transfer in the case of inlet V and outlet inverted V is high. The overall performances for broken ribs are higher compared with the continuous ribs in two-pass cooling channels.

Experimental Investigation of Heat Transfer for a Jet Impinging Obliquely on a Flat Surface

An experimental study investigated the effects of the inclination of an impinging circular jet on heat transfer from a horizontal flat plate. Local Nusselt number distribution was determine depending on inclination angle, jet-to-plate distance, and Reynolds number. The results show that the maximum heat transfer point moves toward the uphill side of the plate and that 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 three specific regions.

Taylor bubble-train flows and heat transfer in the context of Pulsating Heat Pipes

Understanding the performance of Pulsating Heat Pipes (PHPs) requires spatio-temporally coupled, flow and heat transfer information during the self-sustained thermally driven flow of oscillating Taylor bubbles. Detailed local hydrodynamic characteristics are needed to predict its thermal performance, which has remained elusive. Net heat transfer in PHP is contributed by (a) pulsating/oscillating flow (b) distribution of different liquid slugs and bubbles, and, (c) phase-change process; however, its contribution is minimal. In fact, the former two flow conditions are largely responsible for heat transfer in PHPs; such flow conditions can be generated without phase-change and can also be studied independently to observe their explicit effects on PHP heat transfer. With this motivation, systematic experimental investigation of heat transfer is performed during (a) isolated Taylor bubble flow (b) continuous Taylor bubble flow and (c) pulsating Taylor bubble flow, at various frequencies (1 Hz to 3 Hz, as applicable for PHPs) inside a heated square mini-channel of cross-section size 3 mm × 3 mm. This study clearly reveals important insights into the PHP operation. Oscillating Taylor bubbles create significant disturbances in their wake which leads to local augmentation of sensible heat transfer. The implications of bubble length, wake characteristics, oscillating frequency and bubble slip velocity on the heat transfer augmentation and, in turn on thermal performance of PHPs can be clearly delineated from this study. The study also brings out the nuances in the estimation of true bubble slip under time varying Taylor bubble flows.

An investigation of heat transfer between a microcantilever and a substrate for improved thermal topography imaging

This paper reports the numerical and experimental investigation of heat transfer from a heated microcantilever to a substrate and uses the resulting insights to improve thermal topography imaging. The cantilever sensitivity, defined as change in thermal signal due to changes in the topography height, is relatively constant for feature heights in the range 100–350 nm. Since the cantilever-substrate heat transfer is governed by thermal conduction through the air, the cantilever sensitivity is nearly constant across substrates of varying thermal conductivity. Surface features with lateral size larger than 2.5 μm can induce artifacts in the cantilever signal resulting in measurement errors as large as 28%. These artifacts arise from thermal conduction from the cantilever in the lateral direction, parallel to the surface. We show how these artifacts can be removed by accounting for this lateral conduction and removing it from the thermal signal. This technique reduces the measurement error by as much as 26%, can be applied to arbitrary substrate topographies, and can be scaled to arrays of heated cantilevers. These results could lead to improvements in nanometer-scale thermal measurements including scanning thermal microscopy and tip-based nanofabrication.

Experimental investigation of heat transfer in a channel with new winglet-type vortex generators

Abstract Both new punched triangular vortex generators (PTVGs) and punched rectangular vortex generators (PRVGs) have been developed. Both the triangular and rectangular vortex generators were directly punched from the longitudinal winglet at attack angles of 15°, 45° and 75°, respectively. Measurements were carried out for a rectangular channel of an aspect ratio of AR = 2, for a winglet transverse pitch (S) to a longitudinal winglet height (e) ratio of S/e = 0.59, and a winglet height (e) to a channel height (H) ratio of e/H = 0.6. The parameters included the location of the punched vortex generator on the longitudinal winglet, the geometric shapes of the punched vortex generators, and the attached angle of punched vortex generators. The Reynolds numbers considered for the channel flow case (based on the hydraulic diameter) ranged from 3288 to 37,817. The heat transfer results were obtained using the infrared thermal imaging technique. The heat transfer results of the vortex generators were compared with those of a smooth plate. The best heat transfer performance was obtained with the PTVGs. The presence of the vortex generators produced higher heat transfer coefficients than the smooth plate surfaces. Correlations were developed for the averaged Nusselt number for the PTVGs and PRVGs. Results showed a 23–55% increase in heat transfer due to the use of vortex generators. These vortex generators show a more significant increase in heat transfer coefficient for channel flows.

Experimental Investigation of Heat Transfer in Two-phase Flow Boiling

In this article, an experimental investigation is performed to measure the boiling heat transfer coefficient of water flow in a microchannel with a hydraulic diameter of 500 μm. Experimental tests are conducted with heat fluxes ranging from 100 to 400 kW/m2, vapor quality from 0 to 0.2, and mass fluxes of 200, 400, and 600 kg/m2s. Also, this study has modified the liquid Froude number to present a flow pattern transition toward an annular flow. Experimental results show that the flow boiling heat transfer coefficient is not dependent on mass flux and vapor quality but on heat flux to a certain degree. The measured heat transfer coefficient is compared with a few available correlations proposed for macroscales, and it is found that previous correlations have overestimated the flow boiling heat transfer coefficient for the test conditions considered in this work. This article proposes a new correlation model regarding the boiling heat transfer coefficient in mini- and microchannels using boiling number, Reynolds number, and modified Froude number.

Experimental Investigation of Heat Transfer of Single Jet Impingement on a Aluminium Block

Jet Impingement is used in many applications where extensive heating (or) cooling is necessary to produce high heat transfer rate in a localized region. Those applications include glass production, drying of papers, annealing of metals and cooling of electronic equipments. Present work is involved with the experimental investigation of single jet impingement on Aluminium block. The effect of Reynolds number and the distance between the jet and block (H/d ratio) are considered as the interesting variable parameters. The heat transfer rate and reattachment length are reported in detailed for the various Reynolds number and various jet to block ratio. The flow physics revealed that when the Reynolds number increases the reattachment length also increases. The heat transfer rate increases with increase in Reynolds number up to critical heat flux and then further increase of Reynolds number leads to decrease in heat transfer.

Inlet flow effects in micro-channels in the laminar and transitional regimes on single-phase heat transfer coefficients and friction factors

An experimental investigation of heat transfer and pressure drop in rectangular micro-channels was conducted for water in the laminar and transitional regimes for three different inlet configurations. The inlet types under consideration were the sudden contraction, bellmouth, and swirl inlet types, and hydraulic diameters of 0.57, 0.85, and 1.05mm were covered. It was found that the critical Reynolds number and the transitional behaviour in terms of heat transfer coefficients and friction factors were influenced significantly by the inlet type. For the sudden contraction inlet type, which were investigated for both adiabatic, as well as diabatic cases, adiabatic friction factors were predicted well by the laminar Shah and London correlation, while diabatic friction factors were decreased with an increase in wall heat flux. The sudden contraction inlet critical Reynolds numbers were found to be between 1800 and 2000 for adiabatic cases, while for diabatic cases the transition regime commenced at a Reynolds number of about 2000. The bellmouth and swirl inlet types were mostly investigated for diabatic cases only with swirl inlet tests limited to the 1.05mm channel. Laminar friction factors were approximately similar to those of the sudden contraction inlet type, however, after the commencement of transition both inlet types exhibited higher friction factors than the sudden contraction inlet. Minor transition occurred as early as at Reynolds numbers of 1200 and 800 for the bellmouth and swirl inlet types respectively while major transition occurred at Reynolds numbers of approximately 1800 and 1500 respectively. Critical Reynolds numbers were found not be significantly influenced by the channel diameter to length ratio considered in this study. Laminar Nusselt numbers were predicted well by conventional macro-channel thermal entry correlations. The swirl inlet type exhibited the highest friction factors and Nusselt numbers in the transitional regime followed by the bellmouth inlet type. During transition while compared with the sudden contraction inlet, both the bellmouth and swirl inlet types exhibited larger enhancement in heat transfer than increases in the friction factor penalty. Based on the experimental data obtained in this study, a set of correlations were developed which describes the relation between the friction factor and Colburn j-factor. Depending on the inlet type, the correlations predicted between 94% and 100% of the results to within 10% of the experimental measurements.

Experimental investigation of heat transfer from a 2 × 2 rod bundle to supercritical pressure water

Heat transfer experiments with supercritical pressure water flowing vertically upward through a 2×2 rod bundle have been performed at Xi’an Jiaotong University. A fuel-assembly simulator with four heated rods was installed inside a square channel with rounded corner. The outer diameter of each heated rod is 8mm with an effective heated length of 600mm. The experiments covered the pressure range of 23–28MPa, mass-flux range of 350–1000kg/(m2s) and heat-flux range on the rod surface of 200–1000kW/m2. Heat transfer characteristics of supercritical pressure water through the bundle were examined with respect to variations of heat flux, system pressure, and mass flux. These characteristics were shown to be similar to those previously observed in tubes or annuli. The experimental data indicate a non-uniform circumferential wall-temperature distribution around the heated rod. A maximum wall temperature was observed at the surface facing the corner gap between the heated rod and the ceramic tube, while the minimum wall temperature was observed at the surface facing the center subchannel. The difference between maximum and minimum wall temperatures varies with heat flux and/or mass flux. Eight heat transfer correlations developed for supercritical water were assessed against the current set of test data. Prediction of the Jackson correlation agrees closely with the experimental Nusselt number. A new correlation has been derived based on Jackson correlation to improve the prediction accuracy of supercritical heat transfer coefficient in a 2×2 rod bundle.

Experimental investigation of heat transfer from vertical flat surface to upward aqueous foam flow

Paper presents the results of the experimental investigation of the local and average heat transfer process between the vertical flat surface and upward macro foam flow. Macro foam was generated by bubbling gas (air) into the liquid made from the 0.5% concentration of the washing powder (Tide Absolute) in water. Macro foam flow parameters: velocity 0.10÷0.25m/s; volumetric void fraction 0.996÷0.998; bubbles size 0.005÷0.015m. Surface temperature varied from 25 to 40°C; macro foam flow temperature 12.5÷18.0°C.Investigation showed that the heat transfer rate depends on the foam flow parameters: velocity and volumetric void fraction. It was stated that the heat transfer rate increased with the increase of the foam flow velocity and with the decrease of the volumetric void fraction. The local heat transfer rate is lower at the sides and is higher at the middle part of the surface. The maximum of the local heat transfer rate (up to 1137W/(m2K)) is identified at the inlet region. Average heat transfer rate for the whole surface varied from 152 to 331W/(m2K).The results of the experimental investigation were generalized using the criterion equations.