Local Heat Transfer Measurements Research Articles

Influence of thickness of metal foam on the conduction and convection heat transfer for a flat plate with metal foam impinged by a single circular air jet

ABSTRACT The effect of the metal foam thickness on the conduction and convection heat transfer for a metal foam flat plate impinged by a circular air jet is investigated. The IR thermography and thin-metal foil technique are used for the measurement of local heat transfer. An open-cell aluminum metal foam is used for the metal foam flat plate. A 3D-printed resin foam and detached metal foam flat plate are used for the appreciation of the conduction and convection heat transfer. The varying parameters are the thickness of the foam, Reynolds number, and the nozzle exit to plate distance. The presence of the metal foam offers a conduction effect. This predominates over the attenuation in the convective heat transfer by foam due to additional hydraulic resistance. The additional hydraulic resistance offered by the porous foam increases with the increase in the foam thickness. The heat transfer of a porous foamed flat plate decreases with the increase in the foam thickness. The local Nusselt number of the resin foam and detached foam flat plate is almost the same. The conduction effect and attenuation in the convection heat transfer of a metal foam flat plate are quantified by attenuation and enhancement factors. The overall augmentation offered by 4, 8, and 12 mm thick metal foam flat plates is 1.71, 1.42, and 1.43 times compared to the smooth flat plate case, respectively. Hence, it is advisable to use a metal foam flat plate with 4-mm-thick metal foam under circular air jet impingement.

Transient and periodic steady-state characteristics of the local heat transfer measurement by thermal perturbation with Gaussian power density distribution

The local heat transfer coefficient measurement with temperature oscillation induced by periodic thermal perturbation – usually via a Gaussian laser beam, was investigated for the impact of the spikiness (i.e., the standard deviation) elaborated in comparison with the analytical model for dimensional analysis. The statistically more robust technique that relies on the linearity of the spatial phase distribution of the test point array was favored when the target Biot number approaches unity in terms of its order of magnitude. The preferred upper limit for thermographic scanning was discussed as the simplification of later data processing is concerned. Nonetheless, the time elapsed for an acceptable periodic steady state, which in principle leans to the higher end of the target Biot number spectrum in a log scale, indicates the benefit from the time series of pointwise temperature measurement – as in the conventional single-blow testing, where the effect of spikiness, as well as that of the location of individual test point, holds. Note that the vicinity as the target Biot number approaches unity was again observed with higher preference.

Bubble growth on a smooth metallic surface at atmospheric and sub-atmospheric pressure

Bubble growth rate is one of the most important parameters required for the development of accurate mechanistic nucleate boiling heat transfer models. It is also very important for understanding the hydrodynamic forces and the mechanism of bubble departure. This paper presents an experimental study on bubble growth measurements in saturated pool boiling of deionized water on a plain copper surface at atmospheric and sub-atmospheric pressure. The measurements were conducted using a high-speed, high-resolution camera with a microscopic lens. The mechanisms of bubble growth are discussed, while the microlayer evaporation mechanism has been evaluated and discussed using the measured bubble growth curve. The estimated contribution of microlayer evaporation to a single bubble growth is about 70 %, while the contribution of latent heat transfer (evaporation) to the total heat transfer rate from the surface is about 30 %. The remaining 70 % is due to other mechanisms, i.e. conduction and convection. These values were obtained based on the analysis of the bubble growth curve only and agreed with some researchers who conducted local heat transfer measurements using integrated sensors or infrared thermography. These detailed measurement techniques cannot be used with the thick copper block tested in the current study, which was also tested by many researchers in literature and is representative of industrially used surfaces. It was also found that the bubble departure mechanism at atmospheric pressure is due to a static balance between surface tension and buoyancy forces while at sub-atmospheric pressure, it was between buoyancy and liquid inertia forces. The pressure did not have a significant effect on the characteristics of the dynamic contact angle, which was also measured from the instantaneous images of the bubble. It was concluded also that the force balance required for the accurate prediction of departure diameter should be conducted when the two forces are equal, which occurred at time less than the departure time and dynamic contact angle of about 45. In most bubble departure models, researchers recommended the balance to be conducted at the moment of departure when the bubble forms a neck with contact angle of 900 (underestimation to the surface tension force). The analysis of one of the commonly used homogeneous growth models indicated that for homogeneous bubble growth models to be applicable in nucleate boiling, an allowance must be made for the fact that the degree of superheat varies with time during a bubble growth period.

Local Heat Transfer Measurements for an Impinging Synthetic Jet

Abstract Research results demonstrate the heat transfer effectiveness of an impinging synthetic jet toward cooling a plane normal to it. The utility of the synthetic jet lies in that the supply of coolant comes from the device itself as an alternating jetting flow that emerges from a plenum followed by a sink flow that returns to that same plenum. Experiments reported herein were conducted with the synthetic jet driven by an oscillating diaphragm powered by a rotating cam to expel fluid from the plenum out of a single hole, then return it through the same hole. The frequency of diaphragm oscillation and the distance from the synthetic jet's orifice to the surface being cooled are varied in the test program to determine their effects on cooling performance. A numerical study agrees with the results given by the experiment and flow visualization utilizing a smoke generator supports the data and numerical results. The local, time-average Nusselt numbers were measured in the experiment using the thermochromic liquid crystal technique and air as coolant. The color display of each test case was recorded with a fisheye camera. In the case of the highest frequency and shortest distance from orifice to cooled plate, a Nusselt number of nearly 40 was achieved within the central region of the cooled plate when the Reynolds number based upon jet maximum velocity and orifice diameter was 7500 and the distance from the orifice to cooled plate was 3.2 orifice diameters.

Simultaneous high-accuracy measurements of local heat transfer and phase distribution during convective condensation using non intrusive diagnostic tools

An experimental setup was built to measure simultaneously the profiles of the phase distribution and the heat transfer coefficient in a minitube during convective condensation of HFE7000 at low mass velocities. The aim is to overcome the lack of measurements available in the literature in these conditions, mainly due to the difficulty to measure accurately heat transfer and phase distribution simultaneously. The metrology combines two high-precision measurement techniques allowing accurate measurement of the condenser wall temperature by infrared method and the thickness of the liquid film flowing inside the condenser by interferometric or confocal method. A specific protocol was developed to reduce the uncertainty on the wall temperature down to 0.08 °C. The flow regime varies from smooth annular to wavy annular with mean thickness of the liquid film less than 100 μm when the mass velocity is increased from 5 to 30 kg m−2 s−1. The heat transfer coefficient increases locally by up to 30% as the mass velocity increases. These profiles were than compared to the ones of both Nusselt theory and 5 correlations selected from the literature. None of them allow an accurate prediction of the heat transfer coefficient obtained experimentally.

Effects of jet-to-target spacing on primary and secondary peak heat transfer for an obliquely impinging jet

This paper presents an experimental investigation on the primary and secondary peak heat transfer characteristics of an obliquely (45°) impinging jet on a flat and smooth target surface. Local convective heat transfer coefficients have been calculated through a transient liquid crystal thermography technique employing one-dimensional semi-infinite conduction model for the target surface. The effects of jet Reynolds number (Re) and jet-to-target spacings (z/d) have been investigated where Re was varied from 9,000 to 18,000 and z/d was varied from 2.5 to 6. Local heat transfer measurements reveal several important features of oblique jet impingement, e.g., the location of the primary and secondary peaks with respect to the geometric center (GC), its dependence on Re and z/d, and the shape of enhanced heat transfer zone with stagnation, separation, and reattachment features. The two-dimensional heat transfer map is presented in a normalized form Nu/Numax, where Numax is the primary peak heat transfer. It has been found that for a certain z/d, the primary peak occurs at a fixed distance from the geometric center for the range of Re investigated, while the secondary peak location varies with Re. For the smallest z/d, the primary peak heat transfer location was offset by one jet diameter from the GC in the uphill direction, whereas for the moderate z/d = 4, the primary peak occurred near the GC. For the highest z/d, the primary peak was again in the vicinity of GC, however, slight offset on the downhill side. The spent flow reattachment (post the first impingement) which also determined the secondary peak location and heat transfer level was strongly dependent upon the Re. The oblique jet impingement radial variation is also compared against 90° (orthogonal) jet impingement at z/d = 2.5 and 6 at Re = 18,000. It has been observed that the primary peak for above two z/d configurations were sharper for the 45° case when compared to 90°, and the secondary peak was further offset in the downhill direction with higher heat transfer levels for the 45° impingement case compared to 90°.

Simultaneous High-Accuracy Measurements of Local Heat Transfer and Phase Distribution During Convective Condensation Using Non Intrusive Diagnostic Tools

Simultaneous High-Accuracy Measurements of Local Heat Transfer and Phase Distribution During Convective Condensation Using Non Intrusive Diagnostic Tools

Effect of heating on the stability of the three-dimensional boundary layer flow over a rotating disk

The effect of heating on the stability of the laminar three-dimensional boundary layer flow over a rotating disk was experimentally investigated. Local convective heat transfer coefficients were obtained at different running speeds and heating rates by means of an electrically heated disk apparatus placed in a large water tank. The accuracy of the method was assessed by comparison with predictions of the analytical self-similarity solution for laminar flow, and an excellent agreement was found. By means of local heat transfer measurements, the critical Reynolds number corresponding to the onset of vortices was determined as a function of the wall temperature difference and Prandtl number. A substantial increase of the critical Reynolds number with higher wall temperature difference was observed for the three-dimensional flow. The observed stabilizing effect due to heating of three-dimensional water flows was comparable with the predictions of perturbation analyses conducted for two-dimensional flows.

An experimental test of the Mocikat-Herwig theory of local turbulent heat transfer measurements on cold objects

Experimental results are presented of a test of the theory of local turbulent heat transfer measurements proposed by Mocikat and Herwig in 2007. A miniaturized multi-layer heat transfer sensor was developed and employed in this study. The new heat transfer sensor was designed to work in air and liquids, and this capability enabled the simultaneous investigation of different Prandtl numbers. Two basic configurations, namely the flow past a blunt plate and the flow past an inclined square cylinder, were investigated in test sections of wind and water tunnels. Convective heat transfer coefficients were obtained through conventional testing (i.e., employing thoroughly heated test objects) and using the new miniaturized sensor approach (i.e., utilizing cold test objects without heating). The main prediction of the Mocikat-Herwig theory that a specific thermal adjustment coefficient of the employed actual miniaturized heat transfer sensor should exist in the fully turbulent flow regime was proven for developed two-dimensional flow. The observed effect of the Prandtl number on this coefficient was in good agreement with the prediction of the asymptotic expansion method. The square cylinder results indicated the inherent limits of the local turbulent heat transfer measurement approach, as suggested by Mocikat and Herwig.

Experimental Investigation of Local Heat Transfer Measurement Having Inclined Discrete Ribs of Solar Air Heater Duct Using LCT Technique

Abstract The steady-state experiment using liquid crystal thermography has been conducted for the analysis of Nusselt number distribution over the absorber surface of solar air heater (SAH) duct having a gap with the staggered arrangement in inclined rib geometry to recuperate its thermal performance. The heat transfer experiment is performed with uniform heat flux and thermo-chromic liquid crystal (TLC) is utilized to show the temperature distribution profile over the ribbed surfaces of the rectangular duct of aspect ratio of 5. The colored pattern image of TLCs was acquired using a 3CCD (charged couple device) camera and exported to a Tagged Image File Format (TIFF) file format using frame grabbing SOFTWARE SAPERALT which was further processed to get hue, saturation, intensity (HSI) values. The flow parameters considered in this present investigation are Re, d/W, and g/e varied from 4000 − 12,500, 0.15 − 0.45, and 1 − 4, respectively. Experiments has been performed with fixed P/e, r/e, p′/P, α, and e/Dh of 10, 2, 0.6, 60 deg, and 0.0303, respectively. The influence of relative gap position and relative gap width on flow pattern has been analyzed. The maximum augmentation in Nu and f over the smooth duct was obtained as 4.01 and 4.28 times, respectively, at the optimum value of d/W = 0.35 and g/e = 2 under similar flow conditions. The maximum value of THPP obtained at d/W and g/e of 0.35 and 2, respectively, and Reynolds number of 12,445.

Consideration on Local Heat Transfer Measurement of Plate Heat Exchanger with the Aid of Simulation

Abstract
 In this study, the local heat transfer coefficient of boiling and condensation were obtained by an experimental set up using vertical stainless-steel type brazed plate heat exchanger. A series of 8 vertical brazed plates are used as the major components of the test section of experimental set up and are fabricated into layers so that flow channels are formed between the plates through which water and refrigerants are flowing through. The experiments are carried out at the mass flux of 10, 20 and 50 kg/(m2žs). In order to measure the local heat transfer coefficient, flat stainless-steel plates of 10 mm in thickness are installed attached to the vertical plates onto which the thermocouples are positioned to measure the temperature distributions at the surface of the plates. By performing the experiment, the direction of the heat flux across the plate tends to deviate downward especially at the lower part of the plate due to the non-uniform temperature distributions across the plate. The results are analyzed and validated at the mass flux of 10 kg/(m2žs) by the aid of the simulation tool by using ANSYS FLUENT 19.1 to estimate the local heat transfer coefficient and the heat flux across the plate. The analysis result shows that the simulation model can assist to track the deviation of the direction of the heat flow from the horizontal direction across the plate and the experimental results of the local heat transfer coefficient have similar trends with that of the simulation results.

Flow boiling heat transfer of low-GWP working fluids at low mass and heat fluxes in a 4 mm diameter tube

With particular focus on applications to small scale binary cycle systems, the flow boiling heat transfer performances of low-GWP working fluids in a 4 mm diameter tube with vertical upward flow configuration have been investigated based on local heat transfer measurements and visualization of diabatic flow. The working fluids examined in the present study were R245fa, R1233zd(E), R1224yd(Z), and HFE347pc-f. The local heat transfer coefficients (HTC) were measured in a stainless tube with Joule heating while the inlet quality was regulated by preheating. The ranges of the mass and heat fluxes covered 20-40 kg/(m2⋅s) and 6-20 kW/m2, respectively, and the reduced pressure was varied from 0.061 to 0.138. Using a 200 nm-thick layer of transparent aluminum-zinc-oxide (AZO) electrodes deposited by atomic layer deposition (ALD) onto the inner wall of a quartz glass tube, the flow boiling behavior was visualized under diabatic condition. It is shown that the measured HTC of all working fluids examined exhibit strong sensitivity to the heat flux and weak sensitivity to the mass flux. The dominance of the nucleate boiling effect is suggested for the quality of less than 0.5, and the convection boiling effect plays an increasing role for the quality of larger than 0.5 until the dryout incipience. It is quantified that the averaged HTC for different working fluids highly depend on the dryout incipience qualities. The measured HTC profiles were compared to conventional heat transfer correlations, and a possible way to improve the correlation suited in the present low mass and heat fluxes condition has been proposed.

Infrared image processing for local convective heat transfer measurements in rib-enhanced channels

In the context of an experimental study designed to retrieve the local heat transfer characteristics in closed channels with ribs in several configurations by IR thermography, a filtering strategy must be adopted to calculate the laplacian of the temperature field, to remove not only Gaussian noise, but also unwanted local features due to the pattern of the conductors of the heater. In this paper, these issues are addressed with a comparative analysis on filtering and interpolation techniques, in particular mean filtering, local surface fitting and cubic smoothing splines, which are carried out by showing their influence on the smoothed thermogram and on the laplacian of the temperature field. The use of cubic smoothing splines gives the best approximation of the temperature field with respect to the mean filter, which alters the temperature field in proximity of the ribs, and to surface interpolation. However, the only filter among those tried that gives meaningful derivatives on the whole image is the mean filter, with a kernel size equal or bigger to that of the spatial feature to be filtered. The resulting derivative also need further filtering to reduce local spikes induced by the filtering operations.

Experimental and numerical study of oxygen catalytic recombination of SiC-coated material

A combined experimental and numerical approach has been proposed to study the oxygen catalytic recombination coefficient of SiC-coated material for two different types of surface condition: roughened and pre-heated. For the experimental approach, the shock tube facility, which test gas consists of 21% oxygen and 79% argon by volume, is utilized to measure the oxygen catalytic recombination coefficient at near-room surface temperature. The coefficient is deduced from the local heat transfer measurement at the end-wall model of the shock tube via catalytic boundary layer theory. The experimental data indicated that the coefficient for the SiC-coated material varied between 0.0024 and 0.01, depending on the surface condition. The results suggested that even at low surface temperature, both the level of surface roughness and the oxidation due to the pre-heating should be considered carefully for the atomic recombination process. For the numerical approach, the finite rate catalytic modeling is elaborated in detail. The modeling is conducted to better understand the physical interaction for the recombination mechanism. Given the surface reactions considered in the modeling, the catalytic recombination coefficient was not significantly affected by the pressure at low wall temperature. Conversely, the variation in the coefficient was observed to be dependent on the activation energy and the number of active site concentrations at the surface.

Modified surfaces to enhance vertical falling film heat transfer – An experimental and numerical study

In this paper we investigate experimentally and numerically the potential improvement of heat transfer in falling film units by introducing modifications of the heat transfer surfaces. Two different modified surfaces are investigated and the obtained results are compared to the ones acquired on a smooth surface. In our experiments we work with a broad range of operating conditions with Kapitza and Reynolds numbers in the range of Ka = 167–574 and Re 18–423, respectively. We compare local heat transfer measurements conducted at multiple positions and relate them to the film thickness measurements conducted by a laser triangulation scanner. Our experiments identify both cases where the introduction of modified surfaces significantly enhances the heat transfer and conditions where such a measure does not have a significant impact. The observed differences are compared to numerical simulations providing details of the flow structures in the liquid film in different cases. For that purpose, we directly solve the full Navier-Stokes equations in two dimensions, using the volume of fluid (VOF) numerical framework. Our results reveal that the heat transfer can be enhanced by a factor of up to 2.5 due to appearance of significantly enlarged time-dependent recirculation zones occurring behind the surface modifications, as opposed to such zones present in falling film units with smooth heat-transfer surfaces.

Measurements of Local Instantaneous Convective Heat Transfer in a Pipe - Single and Two-phase Flow.

This manuscript provides step by step description of the manufacturing process of a test section designed to measure the local instantaneous heat transfer coefficient as a function of the liquid flow rate in a transparent pipe. With certain amendments, the approach is extended to gas-liquid flows, with a particular emphasis on the effect of a single elongated (Taylor) air bubble on heat transfer enhancement. A non-invasive thermography technique is applied to measure the instantaneous temperature of a thin metal foil heated electrically. The foil is glued to cover a narrow slot cut in the pipe. The thermal inertia of the foil is small enough to detect the variation in the instantaneous foil temperature. The test section can be moved along the pipe and is long enough to cover a considerable part of the growing thermal boundary layer. At the beginning of each experimental run, a steady state with a constant water flow rate and heat flux to the foil is attained and serves as the reference. The Taylor bubble is then injected into the pipe. The heat transfer coefficient variations due to the passage of a Taylor bubble propagating in a vertical pipe is measured as function of the distance of the measuring point from the bottom of the moving Taylor bubble. Thus, the results represent the local heat transfer coefficients. Multiple independent runs preformed under identical conditions allow accumulating sufficient data to calculate reliable ensemble-averaged results on the transient convective heat transfer. In order to perform this in a frame of reference moving with the bubble, the location of the bubble along the pipe has to be known at all times. Detailed description of measurements of the length and of the translational velocity of the Taylor bubbles by optical probes is presented.

Measurements of Local Instantaneous Convective Heat Transfer in a Pipe - Single and Two-phase Flow

Measurements of Local Instantaneous Convective Heat Transfer in a Pipe - Single and Two-phase Flow

Experimental and numerical study of heat transfer in a large-scale vertical falling film pilot unit

Heat transfer in a large-scale vertical falling film pilot unit is investigated experimentally and numerically. We study a broad range of operating conditions with Kapitza and Reynolds numbers ranging from Ka = 167–7010 and Re 18–1854, respectively. We compare local heat transfer measurements, conducted over a vertical length of 4 m, with those obtained by directly solving the full Navier-Stokes equations in two dimensions and using the volume of fluid (VOF) numerical framework. We examine the development region along with the one in which we assume statistically steady conditions. In both our experiments and simulations we see significant differences between the two regions in terms of the magnitude of the heat transfer coefficient. We show how this is a result of the temperature gradient within the liquid film, along with the thickness of the liquid film. The degree of bulk mixing, introduced by the waves, has a profound influence on the thermal boundary layer and depends strongly on the fluid properties and the operating conditions.

Numerical investigation of the effects of discrete guide vanes on the control of heat transfer on the tip surface of a turbine blade

The cooling of the tip surface of a turbine blade is a critical issue with respect to the increase in turbine inlet temperatures. Guide vanes, ribs, pins, dimples, and protrusions have been used by previous researchers in the tip turning region in internal two-pass channels to increase the cooling effect on the tip surface. However, external heat transfer at the tip surface is not uniform. Thus, different local heat transfer measurements are needed for cooling the tip surface. Discrete guide vanes have been suggested to control the cooling effect on tip surfaces. Numerical simulations were performed in six different guide vane cases in a two-pass channel to compare the effects of discrete guide vanes. In a two-pass channel, ribs were installed at 45°. The Reynolds number, which is based on hydraulic diameter and velocity, was fixed at 10,000 in each case. The results indicated that the base case and U-vane case showed high heat transfer at the tip surface of the first channel but low heat transfer at the tip surface of the second channel. Also, the heat transfer on the tip surface was asymmetrical. In comparison, the heat transfer on the tip surface was asymmetrical in discrete guide vane cases. The 1S2S case showed high heat transfer on the tip surface of the second passage pressure side and the 1P2P case showed high heat transfer on the tip surface of the second passage suction side. The arrangement of discrete guide vanes caused different flow characteristics in comparison with the U-vane case. Thus, the design of discrete guide vanes can control heat transfer on the tip surface effectively.

Local heat and mass transfer measurements for multi-layered impingement/effusion cooling: Effects of pin spacing on the impingement and effusion plate

Practical implementation of transpiration cooling system can be realized by multi-layered impingement/effusion cooling system which is composed of two or more layers with pins. Due to restrictions of very low height in multi-layered impingement/effusion cooling, heat transfer characteristics are affected by pin space to diameter. In this paper, the effects of pin space to diameter ratio, sp/d, are investigated on all internal surfaces including impingement plate and pin surfaces. Local heat/mass transfer measured using a naphthalene sublimation method. The ratio of height to diameter ratio, h/d, is fixed at 0.25, and Reynolds number based on the hole diameter is 3000. As a results, heat transfer characteristics on effusion plate are similar to narrow impingement jet cooling without pins, in the case of sp/d=6.0. Local Sherwood number distributions are affected by biased location of pins at sp/d=2 and 1.5. Pins block the flow channel, and make flow area narrower. The secondary peak moved outwards in the radial direction, 18%, and Sherwood number of the secondary peak was enhanced by 6%. Comparing the heat/mass transfer of all of the cases, and considering pumping power, increasing pins (decreasing sp/d) resulted in superior performance. Comparison factor of sp/d=2.0 and sp/d=1.5 was on the 2.2.