Peak Heat Transfer Coefficient Research Articles

Experimental study of flow boiling of FC-72 in fractal-like flow channels

In this study, symmetrical and dichotomous fractal flow channel designs with various configurations in terms of the number of channels being split into smaller channels, were investigated experimentally for their flow boiling heat transfer performance with FC-72 as the working fluid. A multi-channel parallel channel design depicted as Parallel was used as a benchmark. The channels were fabricated by Selective Laser Melting (SLM). Three mass fluxes were studied, viz., 600, 900 and 1200 kg/m2⋅s. The peak heat transfer coefficient and pressure drop were investigated. High-speed visualization studies were also employed to examine the two-phase behaviors in the channels. The fractal flow channels were found to perform comparably with the Parallel channel at low mass fluxes but achieved higher heat transfer coefficients at higher mass fluxes, with the design n = 2, i.e., channel splitting into smaller channels twice, being the highest. The pressure drops of the fractal flow channel designs are higher than the Parallel channel due to their longer channel lengths, and the flow impacting multiple bifurcations at an angle, with n = 4, i.e., channel splitting into smaller channels four times, experiencing the highest pressure drop for all mass fluxes. Visualization studies show that fractal flow channels cause an early annular flow, enhancing heat transfer. Flow reversals in the Parallel channel were observed especially at higher mass fluxes, which suggest no enhancement in the heat transfer coefficient. The fractal flow channels, however, do not experience any flow reversal which could be attributed to the acceleration of the two-phase flow after each bifurcation, which in turn increases the inertia force to overcome the force from vapor expansion. The visualization studies also suggest that increasing the complexity does not result in better flow boiling heat transfer performance. This is due to the multiple flow separations which resulted in no two-phase heat transfer interaction and thus, under-utilization of the entire channel surface area for heat transfer. A comparison with a general correlation for flow boiling gives mean absolute errors of 31.6% and 35.3% for the fractal flow channels and Parallel, respectively.

Experimental and numerical investigations on convective heat transfer of dual piezoelectric fans

An investigation is performed to study the convective heat transfer performance under dual piezoelectric fans. Three main aspects are involved in the current study. Firstly, vibration tests for dual specific piezoelectric fans actuating at the first-mode resonant frequency are conducted to illustrate the influence roles of vibrating phase difference and fan-to-fan pitch on the piezoelectric fan vibration amplitude. Secondly, heat transfer measurements are made to compare the heat transfer among single fan, dual fans in-phase and dual fans out-of-phase. Thirdly, three-dimensional numerical simulations are conducted to reveal the influence mechanism of dual piezoelectric fans on heat transfer. The results show that, the vibrating phase difference of dual fans has nearly no influence on the displacement velocity and amplitude of piezoelectric fan related to single fan once the dimensionless pitch (P/W) is beyond 1.2. The dual piezoelectric fans produce nearly the same peak heat transfer coefficient as that of single fan case. Of particular is that the dual fans operating in-phase produce more favorable heat transfer than the dual fans operating out-of-phase, especially in the gap zone between dual fans. Due to the interaction between dual fans, the streaming flow induced by one vibrating fan suffers the action of sweeping flow of another vibrating fan when they operate out-of-phase. While for the dual fans operating in-phase, the streaming flows induced by vibrating fans merge together to form stronger wall jet flow in the region between two fans.

Global Aeroheating Measurements of Shock–Shock Interactions on Swept Cylinder

The effects of the fin leading-edge radius and sweep angle on peak heating rates due to shock–shock interactions were investigated in the NASA Langley Research Center 20-Inch Mach 6 Air Tunnel. The cylindrical leading-edge models, with radii varied from 0.25 to 0.75 in., represent wings or struts on hypersonic vehicles. A planar oblique shock at 16.7 deg to the flow intersected the fin bow shock, producing a shock–shock interaction that impinged on the fin leading edge. Three fin sweep angles were tested: 0, , and (swept forward). Global temperature data were obtained from the surface of the fused silica fins using phosphor thermography. Metal oil-flow models were used to visualize the streamline patterns for each angle of attack. High-speed zoom-schlieren videos were recorded to show the features of the shock–shock interactions. The temperature data were analyzed using one-dimensional semi-infinite and one- and two-dimensional finite-volume methods. These results were compared to determine the proper heat transfer analysis approach to minimize errors from lateral heat conduction. The dimensional peak heat transfer coefficient augmentation increased with decreasing leading-edge radius. The dimensional peak heat transfer output from the two-dimensional code was about 20% higher than the value from a standard, semi-infinite one-dimensional method.

An Experimental and Numerical Study on the Aerothermal Characteristics of a Ribbed Transonic Squealer-Tip Turbine Blade With Purge Flow

Detailed heat transfer coefficient (HTC) and film cooling effectiveness (Eta) distribution on a squealer-tipped first stage rotor blade were measured using an infrared technique. The blade tip design, obtained from the Solar Turbines, Inc., gas turbine, consists of double purge hole exits and four ribs within the squealer cavity, with a bleeder exit port on the pressure side close to the trailing edge. The tests were carried out in a transient linear transonic wind tunnel facility under land-based engine representative Mach/Reynolds number. Measurements were taken at an inlet turbulent intensity of Tu = 12%, with exit Mach numbers of 0.85 (Reexit = 9.75 × 105) and 1.0 (Reexit = 1.15 × 106) with the Reynolds number based on the blade axial chord and the cascade exit velocity. The tip clearance was fixed at 1% (based on engine blade span) with a purge flow blowing ratio, BR = 1.0. At each test condition, an accompanying numerical study was performed using Reynolds-averaged Navier–Stokes (RANS) equations solver ansys fluent to further understand the tip flow characteristics. The results showed that the tip purge flow has a blocking effect on the leakage flow path. Furthermore, the ribs significantly altered the flow (and consequently heat transfer) characteristics within the squealer-tip cavity resulting in a significant reduction in film cooling effectiveness. This was attributed to increased coolant–leakage flow mixing due to increased recirculation within the squealer cavity. Overall, the peak HTC on the cavity floor increased with exit Mach/Reynolds number.

An experimental study on heat transfer between supercritical carbon dioxide and water near the pseudo-critical temperature in a double pipe heat exchanger

Supercritical carbon dioxide (SCO2) is a promising working fluid for the cryogenic refrigeration, air-condition and heat pump systems. The present study sets up a SCO2–water test loop to study the heat transfer performance of SCO2 in a double pipe heat exchanger. The effects of SCO2-side pressure, mass flux and buoyancy force as well as water-side mass flux are investigated. It is found that the total and SCO2-side heat transfer coefficients reduce as the SCO2-side pressure increases. The peak total and SCO2-side heat transfer coefficients appear at a higher temperature than the pseudo critical temperature. The water-side mass flux has a larger effect on the total heat transfer coefficient compared to the SCO2-side mass flux in the studied cases. The contribution of buoyancy force to the heat transfer performance is large at the small SCO2-side mass flux and it becomes smaller as the SCO2-side mass flux increases. The SCO2-side pressure and water-side mass flux have little effect on the buoyancy force. A heat transfer correlation that includes the effect of buoyancy force is obtained by fitting the experimental data with genetic algorithm.

Experimental determination of convective heat transfer coefficient during controlled frying of potato discs

The convective heat transfer coefficient was measured experimentally using a controlled one-dimensional (1-D) frying methodology. Hollow Teflon disc was used as a sample holder. Thermostable silicon glue was used to seal the sample in the Teflon disc. This insulated the edges of the potato disc from the frying oil thus restricting oil penetration from only the exposed top and bottom surfaces. This rendered this set of frying experiments a 1-D frying process. The peak heat transfer coefficient values were determined to be 3617, 4517 and 7307 W/m2 °C at frying temperatures of 150, 170 and 190 °C, respectively. The heat transfer coefficient reached its peak value towards the end of frying at all temperatures.

One-dimensional numerical study of thermal performance of an organic Rankine cycle system using liquefied natural gas as a cold source for cold energy recovery

A hybrid liquefied natural gas vaporizer system using an organic Rankine cycle for cold energy recovery is proposed for a typical liquefied natural gas (LNG) receiving terminal. Seawater and LNG are used as the hot and cold sources, respectively. A one-dimensional numerical model is established to formulate the conjugated heat transfer and thermodynamic characteristics for the evaporator, condenser, thermolator, and other components of the system. A solution algorithm for the coupled problem is developed, and the temperature distribution profiles for the three working fluids, namely, seawater, propane, and LNG, are obtained. The effects of five typical operating parameters, namely, the inlet temperatures of seawater and LNG, the inlet mass flow rates of seawater and LNG, and the propane condensation pressure, on the system are investigated. In the evaporator, the major thermal resistance occurs in the tube side of propane, and the peak values for the inner side and total heat transfer coefficients are found in the two-phase zone. An inner-side peak heat transfer coefficient for LNG is located near the pseudo-critical temperature in the condenser. The heat transfer coefficients are distributed uniformly on both the inner and shell sides of the thermolator. The seawater inlet temperature and propane condensation pressure are the two most sensitive factors for thermal efficiency. The inlet LNG mass rate and inlet LNG temperature have opposite effects on the thermal efficiency and the organic Rankine cycle output power.

An Exploration of Transport Within Microdroplet and Nanodroplet Clusters During Dropwise Condensation of Water on Nanostructured Surfaces

Experimental studies of dropwise condensation have generally indicated that higher heat transfer coefficients correspond to smaller mean sizes for droplets growing through condensation on the surface. Recent investigations of dropwise condensation on nanostructured surfaces suggest that optimizing the design of such surfaces can push mean droplet sizes down to smaller values and significantly enhance heat transfer. This paper summarizes a theoretical exploration of the limits of heat transfer enhancement that can be achieved by pushing mean droplet size to progressively smaller sizes. A model analysis is developed that predicts transport near clusters of water droplets undergoing dropwise condensation. The model accounts for interfacial tension effects on thermodynamic equilibrium and noncontinuum transport effects, which become increasingly important as droplet size becomes progressively smaller. In this investigation, the variation of condensing heat transfer coefficient for droplet clusters of different sizes was explored for droplet diameters ranging from hundreds of microns to tens of nanometers. The model predictions indicate that the larger droplet transport trend of increasing heat transfer coefficient with decreasing mean droplet size breaks down as droplet size becomes smaller. The model further predicts that as drop size becomes smaller, a peak heat transfer coefficient is reached, beyond which the coefficient drops as the size continues to diminish. This maximum heat transfer coefficient results from the increasing importance of surface tension effects and noncontinuum effects as droplet size becomes smaller. The impact of these predictions on the interpretation of dropwise condensation heat transfer data, and the implications for design of nanostructured surfaces to enhance dropwise condensation are discussed in detail.

Planar vapor chamber with hybrid evaporator wicks for the thermal management of high-heat-flux and high-power optoelectronic devices

Heat spreaders based on compact vapor chambers offer one attractive approach to the thermal management of high-power electronics. We report our design and experimental characterization of advanced evaporator wicks and thin planar vapor chambers incorporating these wicks. The hybrid wicks combine distributed high-permeability liquid supply structures with thin (monolayer) evaporation layers to achieve both low thermal resistance and high limiting heat fluxes over large heating areas. We model and experimentally characterize the capillary and heat transfer performance of liquid spreading layers consisting of mono-layers of Cu particles and identify a range of optimal particle diameters maximizing their performance. The thin liquid spreading layers are integrated with three different types of liquid supply structures, namely, columnar arteries, converging lateral arteries, and bi-porous structures. The resulting hybrid wicks show comparable heat transfer performances with critical limiting heat fluxes >350W/cm2 over heating areas of 1cm2 and peak heat transfer coefficients >20W/cm2K. These results confirm the effectiveness of our hybrid wick designs and also that evaporation heat transfer is dominated by the liquid spreading layers. A prototype vapor chamber incorporating CTE-tailored envelopes and the hybrid wick is developed for potential applications in the thermal management of laser diode arrays. We demonstrate an evaporator resistance of approximately 0.075K/(W/cm2), while removing over 1500W from a 4cm2 heating area.

Enhancement of Heat Transfer Characteristics of Transformer Oil by Addition of Aluminium Nanoparticles

Abstract A two step approach involving the synthesis of Al nanoparticles by mechanical milling followed by dispersion of the nanoparticles in the base fluid is adopted in the present work to prepare transformer oil based nanofluids. Stainless steel (AISI 304) probes of diameter 15 mm and height 70 mm were used to determine the cooling rate intensities of nanofluids. Heat transfer coefficients were determined using Kobasko’s method. A dynamic contact angle analyzer was used to determine the contact angle of the droplet on the substrate. The addition of Al nanoparticles to the base fluid decreases the wettability and improves its heat transfer capability. The vapour phase stage existed for a longer period of time for transformer oil than for Al-transformer oil based nanofluids. The dispersion of nanoparticles in the base fluid is believed to disrupt the vapour blanket stage in the early stage of the cooling process. The peak heat transfer coefficient increases with an increase in the Al nanoparticle content in the oil. The addition of 0.5 vol % nanoparticles enhances the peak heat transfer coefficient by about 70 %.

Refrigerant flow boiling heat transfer in parallel microchannels as a function of local vapor quality

Flow boiling of refrigerant HFC-134a in a multi-microchannel copper cold plate evaporator is investigated. The heat transfer coefficient is measured locally for the entire range of vapor qualities starting from subcooled liquid to superheated vapor. The test piece contains 17 parallel, rectangular microchannels (0.762 mm wide) of hydraulic diameter 1.09 mm and aspect ratio 2.5. The design of the test facility is validated by a robust energy balance as well as a comparison of single-phase heat transfer coefficients with results from the literature. Results are presented for four different mass fluxes of 20.3, 40.5, 60.8, and 81.0 kg m −2 s −1, which correspond to refrigerant mass flow rates of 0.5–2.0 g s −1, and at three different pressures 400, 550 and 750 kPa corresponding to saturation temperatures of 8.9, 18.7, and 29 °C. The wall heat flux varies from 0 to 20 W/cm 2 in the experiments. The heat transfer coefficient is found to vary significantly with refrigerant inlet quality and mass flow rate, but only slightly with saturation pressure for the range of values investigated. The peak heat transfer coefficient is observed for a vapor quality of approximately 20%.

Digital computer simulation of dropwise condensation from equilibrium droplet to detectable size

A computer simulation of dropwise condensation of steam on a 240×240 μm surface with 60,000 randomly spaced nucleation sites is described. The maximum drop radius achieved was 3.9 μm, 0.21 ms after the start of condensation. Uniform radii drop generations noticed by Rose and Glicksman (1973) had not developed completely by 0.21 ms, although predicted to do so. The characteristic profile on the drop size histogram predicted by Tanaka (1979) extends to drop sizes smaller than have been observed in experiments. A peak heat transfer coefficient of just over 2 MW/m 2K was obtained, about twice the value measured by Tanasawa et al. (1978) immediately after the condensing surface had been wiped.

Heat transfer from vertical surfaces to dense gas-fluidized beds

Heat transfer to beds of solids fluidized by gas has been measured from bed walls and from small diameter heated cylinders immersed in the fluidized solid. The experiments have been carried out in beds of 90 mm and 290 mm dia., for different bed depths, for different locations of the heating surface within the bed and at the walls. The size of particles was varied from 100 μm to 1 mm in diameter, in three materials, glass, Diakon, and nickel. Four different distributors were examined. By varying the position of the heating surface at the walls it has been found that there is a significant variation in heat transfer with distance from the distributor. By varying the diameter of the bed it has been found that peak heat transfer coefficients are greater in the smaller diameter bed, but the dependence of heat transfer coefficient upon velocity peaks sharply in the smaller diameter bed with a more gradual change in the larger bed. The experimental measurements have been expressed in the form of a correlation of Nusselt upon Archimedes group, and upon a thermal capacity group. The dependence of Nusselt upon the thermal capacity group is much greater than previously reported.

Cooling of a moving plate with an impinging circular water jet

A hybrid scheme combining experimental and numerical methods has been developed to study heat transfer in the case of a circular jet impinging on a moving metal plate. The experiment is intended to resemble the cooling system in steel rolling. Heat transfer characteristics with temperatures above and below boiling are investigated. Heat transfer due to jet impingement on a stationary plate is first studied to verify the present method. The effect of the plate motion is to elongate the water layer in the moving direction, resulting in heat transfer enhancement downstream of the stagnation point. Peak heat transfer coefficients are almost the same for all cases studied, because the plate temperatures at the impingement zone drop rapidly below the boiling temperature, resulting in single-phase forced convection. Overall cooling efficiency is better for the moving case than for the stationary case.

Method and apparatus for measuring heat transfer distributions on moving and stationary plates cooled by a planar liquid jet

An experimental method and apparatus were developed to measure local heat transfer distributions on moving and stationary plates. The apparatus can be used to study the effect of surface motion where a flat plate geometry is important. Such cases arise when different heat transfer regimes, such as single-phase forced convection, nucleate boiling, and film boiling, occur in close proximity on moving strips and plates cooled by planar liquid jets. In these cases, surface motion is either in or opposite to the flow direction. Versatility is an important feature of the apparatus because both moving and stationary plates can be considered. Surface temperatures of a test specimen were measured and used with a numerical solution of the energy equation for the specimen to determine local heat transfer coefficients in the vicinity of a planar jet. Experimentes were performed for plate speeds less than one-half of the impingement velocity. For single phase forced convection, heat transfer coefficients near the stagnation point were not appreciably affected by surface motion. In addition, transition to a turbulent boundary layer was delayed when the surface motion was in the flow direction. In nucleate boiling, the peak heat transfer coefficient was shifted in the direction opposite to the plate motion where surface temperatures were higher.

The effects of varying composition on heat transfer in the critical region of binary fluid mixtures

In the present study, the refrigerants R-115 and R-22 were combined in varying proportions. It was seen that the free convective heat transfer from a horizontal wire was enhanced in the near critical region for all cases. For near critical conditions, peak heat transfer coefficients showed steady improvement as the percentage of R-22 was increased. As the composition is varied, maximum values of the heat transfer coefficients obtained for small temperature differences ranged from 340 to 690 percent higher than those values found larger temperature differences.

Jet-Impingement Heat Transfer for a Circular Jet Impinging in Crossflow on a Cylinder

Quasi-local heat transfer coefficients were measured on a cylinder on which a circular jet impinged in crossflow. The jet diameter and the distance between the initiation of the jet and the cylinder surface were varied parametrically, as was the jet Reynolds number. The measurements showed that the axial distribution of the heat transfer coefficient peaked at the impingement point. For a fixed jet diameter and Reynolds number, the peak heat transfer coefficient increased as the distance between the initiation of the jet and the cylinder surface decreased. The peak coefficient also increased with decreasing jet diameter at fixed values of the jet initiation distance and the Reynolds number. User-oriented correlations of the peak Nusselt number with the geometrical parameters and with the Reynolds number were derived. The dropoff of the heat transfer coefficient with increasing axial distance from the impingement point was more rapid for smaller jet initiation distances (at a fixed jet diameter and Reynolds number) and for smaller jet diameters (at a fixed initiation distance and Reynolds number). The impingement pattern was examined by flow visualization.

Impingement of a circular jet with and without cross flow

Measurements are reported for the local heat transfer to an impinging air jet with and without a cross flow of air. At large jet-to-plate spacing the cross flow diminishes the peak heat transfer coefficient ; at a smaller spacing cross flow can increase the peak heat transfer coefficient. A correlation is obtained for the average heat transfer in the absence of cross flow.

Effect of Uncooled Inlet Length and Nozzle Convergence Angle on the Turbulent Boundary Layer and Heat Transfer in Conical Nozzles Operating With Air

Nozzle boundary layer and heat transfer measurements are presented for nozzles having half angles of convergence of 30 and 60 deg, each operating in conjunction with a short and long uncooled pipe inlet. The long inlet, which produced a fully developed turbulent boundary layer at the nozzle entrance, did not significantly alter the nozzle heat transfer distribution relative to values obtained with the short pipe inlet. Peak heat transfer coefficients for the high convergence nozzle were nearly 40 percent higher than the peak value for the 30 deg half angle of convergence nozzle. Measured heat transfer coefficients were compared to predictions based on a boundary layer theory and a pipe flow type correlation.