Measured Heat Transfer Coefficients Research Articles

Heat Transfer Coefficient Distribution on Oil Injection Cooled Gears: Experimental Method, Uncertainty, and Results

Abstract To design gearboxes with very high power densities, an effective means of cooling the gears and knowledge of the achievable heat transfer coefficients are necessary. In this paper, a method to measure heat transfer coefficients for oil injection cooled gears is presented. Contrary to other experimental investigations, a single hollow spur gear is used. To measure heat transfer coefficients, a temperature gradient between the gear and the hot oil needs to be induced. This is achieved by injecting hot oil at realistic temperatures and cooling the inside diameter of the gear. This enables the measurement of heat transfer coefficients in absence of any dissipative or frictional losses, decreasing the measurement uncertainty. In addition, the novel method yields spatially resolved HTC data. The uncertainty of the method is assessed using Monte Carlo simulations. Experimental results for various operating conditions are presented. For all investigated oil flowrates, the same characteristic behavior of the average heat transfer coefficient versus rotational speed was observed. This observation can be explained by using a kinematic model of the oil jet. The geometry of the gear and the cooling arrangement and the spatially resolved HTC data presented in this paper provide a complete basis for the validation of numerical simulations.

Effect of electrohydrodynamic (EHD) forces on charging of a vertical latent heat thermal storage module filled with octadecane

The effect of Electrohydrodynamic (EHD) forces on the charging speed of a vertical latent heat thermal storage module filled with Octadecane as the Phase Change Material (PCM) was investigated experimentally. The melting process was initially governed by natural convection before introducing EHD forces. The melting process was studied under different applied voltages and the interaction between the EHD forces and gravitational forces (natural convection) was visualized using a novel shadowgraph measurement technique.It was found that the EHD forces significantly reduced the charging speed by up to 1.7 times when applying +6 kV DC for a Rayleigh number of (Ra=9×107) and a Stefan number of (Ste=0.045). The level of enhancement was increased by applying higher voltages, with cell bifurcation being identified as the main mechanism of enhancement. Local heat transfer coefficient measurements from the shadowgraph system identified the different heat transfer regimes: electrically dominated regime and mixed electrical and gravitational heat transfer regimes. Finally, the findings revealed that EHD forces expedited convection heat transfer by reducing the size of the convection cell.

Heat Transfer Characteristics of Latent Heat Thermal Energy Storage for Floating Concentrate Solar Power Systems

The heat transfer characteristics of the latent heat thermal energy storage (LHTES) for floating concentrated solar power (CSP) systems were investigated experimentally and numerically. The understanding of natural convection heat transfer between the heat transfer fluid (HTF) and a tube bundle in the heat exchanger is important for the thermal design of the LHTES. As a primitive study, the analytical model of natural convection heat transfer was developed between the HTF and the tube bundle in the heat exchanger of LHTES. In this study, D-mannitol was used as the HTF for the LHTES. To simplify the tube bundle of the heat exchanger, a heated cylinder was installed in the liquid D-mannitol. In the experiment, the average temperature of the cylinder and the heat flux were measured, and the heat transfer coefficient under the natural convection was obtained. Meanwhile, the numerical analysis of natural convection heat transfer was conducted using the commercial computational fluid dynamics (CFD), PHOENICS. The temperature and velocity profiles around the heated cylinder were obtained in the numerical simulation. The heat transfer coefficient was compared with the author’s experimental data. The numerical result was in good agreement with the measured heat transfer coefficient.

Heat transfer in the falling liquid film on an array of horizontal tubes with MAO coating

The paper presents investigation results on heat transfer of falling liquid films of Freon R21 on an array of horizontal tubes with porous Al2O3 coating obtained by the method of microarc oxidation (MAO coating). The microcharacteristics of samples (porosity, thickness, surface roughness) were measured. The results of measuring heat transfer coefficients on the surface of an aluminum tube with a MAO coating and a reference smooth tube were compared. Heat transfer on the surface of the tube with a porous coating was lower than on a smooth reference tube. It was shown that to intensify heat transfer on a surface with a MAO coating, it is necessary to increase the porosity of the coating and decrease its the thermal resistance.

Heat transfer coefficients of additively manufactured tubes with internal pin fins for supercritical carbon dioxide cycle recuperators

This paper describes the measurement of convective heat transfer coefficients and friction factors for sCO2 flowing in additively manufactured tubes with internal pin fins at the US DOE’s National Energy Technology Laboratory in Morgantown, WV. The measurement procedures were validated by conducting benchmark tests with smooth stainless-steel tube and comparing the results with published correlations for Nusselt number (Nu) and friction factor. Over Reynolds numbers (ReD) ranging from 5 × 104 to 2.5 × 105, measured Nu was within 5% of the Dittus-Boelter correlation and measured friction factors were within 5% of the McAdams correlation for smooth tube flow.The candidate pin fin patterned pipes were additively manufactured (AM) at the Oak Ridge National Laboratory. The pins were circular or elliptical in cross-section and were printed at a 30° angle relative to the inner wall (to meet AM constraints). The pin arrangement was helical to promote enhanced heat transfer due to swirl flow. Pin length to diameter aspect ratio was 1.33, 2, and 8, while the pin diameter to tube diameter ratio was 0.188, 0.125, and 0.063. Tests were performed for ReD varying from 6.9 × 104 to 2.2 × 105 and at conditions equivalent to the low pressure (LP) outlet (8.69 MPa, 361 K) and the high pressure (HP) inlet (20.7 MPa, 350 K) of the low temperature recuperator (LTR) in an indirect sCO2 cycle. The Wilson plot technique was utilized to measure the bulk heat transfer coefficients.For the best performing design (tube A, pin length to tube diameter ratio: 1.33, pin diameter to tube diameter ratio: 0.19), the local heat transfer coefficient increased by 136% relative to the Dittus-Boelter correlation at the LTR low pressure outlet and 194% at the LTR high pressure inlet. These correspond to a 282% and a 271% increase in the product of the heat transfer coefficient and surface area (adjusted for fin efficiency) product, respectively. Large pressure drops across the test articles were observed. For Tube Design A, the average friction factor, across the range of ReD considered, was significantly larger than the McAdams correlation at both the LTR LP outlet and the LTR HP inlet. A thermal performance factor was utilized to express the ratio of material required to build a finned heat exchanger relative to a finless heat exchanger with the same heat duty and pumping power. Tube Design A was estimated to decrease the required heat exchanger material by 13%.

Experimental study of heat transfer characteristics of finned-tube and circular-pore heat exchangers in oscillatory flow

This work is concerned with an experimental investigation of the thermal performance of two thermoacoustic heat exchangers characterized by different pore geometries, namely a circular-pore geometry and a finned-tube geometry. A standing wave engine, where the heat exchangers under test play the role of ambient HXs, is used as test-rig. Heat transfer rates measurements by standard energy balance techniques and dynamic pressure measurements are used to assess the impact of the two heat exchangers on the engine performance. The gas-side heat transfer coefficient, expressed as Nusselt number, is also determined for the finned-tube heat exchanger. The resulting values are compared to the heat transfer coefficients estimated in analogous experimental studies and by predictive models. Results show that the circular-pore heat exchanger reduces the performance of the engine compared to the finned-tube heat exchanger by about 23%, being affected by higher thermal and viscous irreversibility. Moreover, the boundary layer conduction model exhibits a better agreement with the measured heat transfer coefficients compared to other models. A new correlation law, based on regression of the experimental data, is also derived.

Heat transfer during film condensation inside horizontal tubes in stratified phase flow

In this paper, the experimental study of heat transfer during condensation of freons R22 and R407С in a plain smooth tube with 17 mm inner diameter was carried out at saturated condensing temperature 40 °C, while mass velocity ranged between 6 and 57 kg/(m2s) and vapour quality changed from 0.23 to 0.95. The unique measurements of circumferential heat fluxes and heat transfer coefficients were performed with the thick wall method during the stratified flow of the phases. The authors performed numerical simulation of heat transfer from condensing vapour to cooling water through the thick-walled cylindrical wall. The CFD model was validated by conducting the physical experiment, which indicated the results coincidence with an error from 7 to 20%. The obtained results allowed improving prediction of effective heat transfer coefficients for vapour condensation, which takes into account the influence of condensate flow in the bottom part of the tube on the heat transfer. This method generalizes with sufficient accuracy (error ± 30%) the experimental data on condensation of freons R22, R134a, R123, R125, R32, R410a, propane, isobutene, propylene, dimethyl ether, carbon dioxide and methane under the stratified flow conditions. Using this method for designing heat exchangers, which utilize such types of fluids, will increase the efficiency of thermal energy systems.

Experimental and Numerical Study of Chord-Wise Eight-Passage Serpentine Cooling Design for Eliminating the Coriolis Force Adverse Effect on Heat Transfer

Abstract Gas turbine blades are equipped with internal cooling channels which are connected by 180 deg bends. Due to combined effects of Coriolis force and centrifugal buoyancy force, the heat transfer increases on the trailing side (pressure side) and decreases on the leading side (suction side) for radially outward flow. The trend in heat transfer is opposite for radially inward flow. This configuration leads to nonuniform blade temperature which in unfavorable for blade lifespan. This paper presents a novel eight-passage serpentine design, where passages are arranged along the chord of the blade, to rectify the negative effects of Coriolis force on heat transfer and is an extension four- and six-passage smooth channel studies conducted by the authors earlier. Transient liquid crystal thermography (TLCT) is carried out for detailed measurement of heat transfer coefficients. Heat transfer experiments were performed for Reynolds numbers between 14,264 and 83,616 under stationary conditions. For experiments under rotation, non-dimensional Rotation number is set as 0.05. Heat transfer enhancement levels of nearly twice the Dittus–Boelter correlation (for developed flow in smooth tubes) are obtained under stationary conditions. Under rotation, it is seen that the heat transfer enhancement levels on the leading and trailing sides are similar to each other and also with the stationary condition. Some differences in heat transfer are observed on local level, when rotation cases are compared against the stationary cases. Numerically predicted flow field is presented to support the experimental findings.

Steady-state transverse heat transfer in a single channel CICC

Current mathematical models and numerical codes used for thermal–hydraulic analyses of forced-flow superconducting cables used in the fusion technology, such as, e.g. Cable-in-Conduit Conductors (CICCs), are typically 1D. They demand reliable constitutive expressions for the transverse momentum, energy and optionally also mass transport processes occurring between different parallel conductor components, in order to reliably simulate the behavior of any superconducting magnet design either at normal operating conditions or during a quench evolution. Energy transport processes can effectively be expressed using heat transfer coefficient (HTC) and the considered wetted perimeter/surface. Only few HTC correlations for flow in a CICC bundle have been proposed in the literature, but none of them is widely accepted for predictive use. As a result, in thermal-hydraulic studies of conductors designed for the EU-DEMO coils standard heat transfer correlations for flows in smooth tubes are utilized, although they do not seem very appropriate for CICCs. Systematic measurements of heat transfer coefficients in a CICC bundle should be conducted to provide an experimental database for further trials to develop a reliable predictive heat transfer correlation. In the present work we report the results of measurements of the steady-state HTC between a jacket wall and demineralised water flowing (i) in a smooth tube and (ii) in a CICC bundle of the reference sample (JT60-SA TF conductor). The measurements were carried out at the THETIS installation. We used four values of water inlet temperature, in the range 30–68 °C, and various mass flow rates, in the range 0.15 – 0.40 kg/s, to obtain the results in possibly wide range of Pr and Re numbers. The experimental values of HTC for CICC slightly increased with the mass flow rate from about 7500 to 9400 W/(m2∙K) in the considered Re range (890–3490) and were almost independent on temperature. The performed experiment was also simulated with the THEA code for additional validation of the obtained HTC values.

Flat miniature heat pipe with sintered porous wick structure: experimental and mathematical studies

Miniature heat pipes are considered as an innovative solution able to dissipate high heat fluxes with low working fluid charge, temperature control, and operating with minimum energy consumption and less noise. In this paper, results show that tilt ranging [10°:60°] has not an impact on heat transfer meanwhile the system thermal resistance is the lowest at horizontal position. A theoretical analysis on heat pipe thermal performance has been carried out based on experimental data by predicting heat pipe evaporation and condensation heat transfer coefficient. The measured heat transfer coefficients are close to the predicted results with a mean deviation less than 10%. A hydraulic mathematical model is developed and shows a good agreement with Kaya's study using water as working fluid. This work comprises a parametric study that investigates the effect of particles size, porosity, permeability, heat pipe length on heat transfer and capillary pressure. The obtained results show that the heat pipe operating temperature rises when particles size increases from 1μm to 90 μm and the optimum porosity range is between [10%–40%].

Integrative numerical modeling and thermodynamic optimal design of counter-flow plate-fin heat exchanger applying neural networks

In this study, an optimization technique combined with a Random Vector Functional Link (RVFL) network in the form of a surrogate-assisted approach was carried out for the optimal design of counter-flow plate-fin compact heat exchanger (PFHE) with offset-strip fins considering different mass flow rates. Computational Fluid Dynamics (CFD) simulations generated dataset for training and testing by RVFL network, reducing processing time and assuring PFHE accuracy when applied in many-objective optimization based on Non-Dominated Sorting Genetic Algorithm III (NSGA-III). Next, the NSGA-III was designed to achieve the maximum effectiveness, minimum volume, and pressure drop at the hot and cold sides in PFHE design. For experimental validation the Shear Stress Transport (SST) k-ω turbulence model was used to simulate the flow and heat transfer in PFHE showing 4.36% and 3.27% of error, respectively, to pressure drop and convective heat transfer coefficient measurements. The optimized results by NSGA-III indicated that the volume and effectiveness values are in agreement with the literature, decreasing approximately 55.4% and 72.3% of pressure drop at the hot and cold sides, respectively, which showed that the convective heat transfer was enhanced and the flow resistance was also significantly reduced.

A new method for heat transfer coefficient measurements of single-phase fluids during laminar flow in microchannels

The measurement of heat transfer coefficients in microchannels is complicated due to the small sizes involved. Moreover, a heat transfer mechanism which is not usually considered, the axial conduction effect in the channel wall, must also be evaluated in micro-scale measurements. Previous heat transfer coefficient measurements have not accounted for the axial conduction effect, and those measurements showed the inconsistent result with the theory. In this paper, a new measurement method is developed to validate the theory that predicts a Nusselt number independent of Reynolds number in the laminar flow regime for microchannels. A numerical model is used to simulate heat transfer characteristics in a microchannel with wall conduction, and to predict the wall temperature difference between a location at the end of the heater and a location 3 mm away from the heater on the microchannel. The temperature difference is experimentally measured on a 160 μm hydraulic diameter microchannel and compared with the numerical model. The comparison shows that the Nusselt number in the laminar flow regime for the microchannel is independent of Reynolds number at least down to Re=300.

Novel in situ measurement methods of the total heat transfer coefficient on building walls

The performance gap between the design and the as-built energy performance of buildings is considered a significant risk that may compromise the efforts on meeting the carbon reduction targets set by governments in Europe. The relative importance of irregularities or defects within building walls is more prominent when the thermal insulation level increases. It is thus necessary to quantify the global thermal insulation level of building walls (i.e., including thermal bridges). Classical contact measurement methods only allow local measurements and would thus require the use of a high number of sensors which would limit their applicability. Besides, the presence of thermal bridges prevents the use of contact heat flux measurements because of the 2D or 3D nature of the associated heat transfers through the building walls. To overcome these limitations, the present study proposes an alternative approach consisting in measuring the heat flux on one location and extrapolating its value on the whole building wall through the measurement of the total heat transfer coefficient. Therefore, we developed and/or further improved five different in situ measurement methods of the total heat transfer coefficient on building walls. The measurement methods were thoroughly tested indoor thanks to lab-scale experiments carried out both in steady-state and transient regimes. The results obtained demonstrated the accuracy and the robustness of the proposed methods, as well as their intrinsic limitations. These in situmethods can be used to assess the heat losses on a whole building wall, including the contribution of thermal bridges.

Measurement of Convective Heat Transfer Coefficients With Supercritical CO2 Using the Wilson-Plot Technique

Abstract This paper describes the measurement of convective heat transfer coefficients and friction factors for sCO2 flowing in a smooth tube and compares the results with published correlations for validation. The paper also describes the Heat Exchange and Experimental Testing (HEET) rig recently designed and built at the U.S. Department of Energy’s (DoE’s) National Energy Technology Laboratory (NETL) in Morgantown, WV. The Wilson-plot technique used for measuring the heat transfer coefficients is described along with the data reduction process. The Wilson-plot technique was chosen as the basis for the design of NETL’s HEET rig. Advantages of the Wilson-plot technique include the (1) ability to measure high convective heat transfer coefficients accurately, (2) ability to measure average heat transfer coefficient for complicated heat exchange geometries like those produced using additive manufacturing, (3) ability to measure heat transfer coefficients on both sides of a heat exchanger independently, and (4) simplicity of experimental setup. Capabilities of the HEET rig include pressure to 24 MPa (3500 psig), temperature to 538 °C (1000 °F), mass flow rate to 1.5 kg/s (3 lb/s), and Re to 500,000. The rig is designed to operate with pure CO2 or a mixture of CO2 and up to 10% N2 by volume to study the impact of gas mixtures typical of direct-fired sCO2 power cycles on the convective heat transfer and pressure drop. Preliminary tests in the HEET rig were performed with smooth stainless-steel tube and pure CO2, and the results were compared with published correlations for Nusselt number (Nu) and friction factor. Over a Reynolds number (Re) range from 58,000 to 228,000, measured Nu was compared to predictions using the Dittus and Boelter equation (Kreith and Bohn, 1993, “Principles of Heat Transfer, West Publishing Company”) within 5% and measured friction factors were compared to predictions using the McAdams correlation (“McAdams, 1954, “Heat Transmission,” 3rd ed., McGraw Hill, New York)” for smooth tube to be within 5%.

Heat transfer of R134a in a horizontal internally ribbed tube and in a smooth tube under super critical pressure

The present study compared measured heat transfer coefficients for supercritical pressure R134a in a horizontal internally ribbed tube (din = 15.5 mm) and in a smooth tube (din = 16 mm) for mass fluxes, G, from 100 to 350 kgm−2s−1, heat fluxes, q, from 10 to 25 kWm−2, pressures, P, from 4.26 to 5 MPa and inlet temperatures from 70 to 105 °C. The heat transfer coefficients, buoyancy effects and pressure drop are compared for the various operating conditions. The results show that the tube wall temperature in the internally ribbed tube varied less for the various working conditions than in the smooth tube. Comparisons of the measured heat transfer coefficients for all 4800 experimental data points showed that the heat transfer coefficients in the internally ribbed tube are 4.61 times those in the smooth tube on the top and 1.40 times those in the smooth tube on the bottom. The buoyancy threshold in the internally tube is higher than in the smooth tube for both the Grq/Grth and GrbReb2.0ρbρwxd criteria which shows that the buoyancy in the internally ribbed tube is suppressed. Moreover, the pressure drop in the internally ribbed tubes is higher than in the smooth tubes for the same working conditions.

Measurement of heat transfer coefficients for steam condensation on a vertical 21.5-mm-O.D. tube in the presence of air

ABSTRACT Governing the rate of heat transport by condenser tubes in the passive containment cooling system (PCCS), the steam condensation over a vertical cylinder in the presence of air was investigated experimentally. The main objective of this study was to explore if the condensation heat transfer coefficient relies on the tube dimension, which has been a variable missed in most condensation models or has been embraced without experimental demonstration under phase change environments. The mean heat transfer coefficient was measured in the condensation test facility named JERICHO (JNU Experimental Rig for Investigation of Condensation Heat transfer On tube). The outer diameter of the condenser tube used in this study was set to 21.5 mm. The measured heat transfer coefficients were compared to those obtained from the 40-mm-O.D. tube, and a multiplier to correct the variation of the heat transfer coefficient with the tube diameter was proposed for its application to Lee correlation. The proposed correlation was further validated against another set of experimental data obtained from a separate test facility housing the 31.8-mm-O.D. tube.

NH3 condensation in a plate heat exchanger: Experimental investigation on flow patterns, heat transfer and frictional pressure drop

This paper investigates NH3 condensation in a plate heat exchanger by visualizing the flow patterns and measuring heat transfer coefficients and frictional pressure drop. Visualization experiments were conducted between 20 and 100 kgm−2s−1. Full film flow takes place at large mass fluxes and intermediate mass fluxes of low vapor qualities, while partial film flow occurs at small mass fluxes and intermediate mass fluxes at high vapor qualities. The heat transfer and frictional pressure drop experiments cover the mass fluxes of 21~78 kgm−2s−1, the averaged vapor qualities of 0.05~0.65 and the saturated pressure of 630 to 930 kPa. Vapor qualities have significant influences on heat transfer and frictional pressure drop. In the tested ranges, the effect of mass fluxes is noticeable on frictional pressure drop, but is moderate on heat transfer. The impact of saturated pressure is small. The heat transfer reflects the change of flow patterns. The frictional pressure drop shows the characteristics of separated flow.

Measurement and Validation of Heat Transfer Coefficient between Molten Oxygen Free Copper and Mold

Measurement and Validation of Heat Transfer Coefficient between Molten Oxygen Free Copper and Mold

Measurement of Convective Heat Transfer Coefficients of Horizontal Thermal Screens Under Natural Conditions

Convective heat transfer is the main component of greenhouse energy loss because the energy loss by this mechanism is greater than those of the other two components (radiative and conductive). Previous studies have examined the convective heat transfer coefficients under natural conditions, but they are not applicable to symmetric thermal screens with zero porosity, and such screens are largely produced and used in Korea. However, the properties of these materials have not been reported in the literature, which causes selectivity issues for users. Therefore, in this study, three screens having similar color and zero porosity were selected, and a mathematical procedure based on radiation balance equations was developed to determine their convective heat transfer coefficients. To conduct the experiment, a hollow wooden structure was built and the thermal screen was tacked over this frame; the theoretical model was applied underneath and over the screen. Input parameters included three components: 1) solar and thermal fluxes; 2) temperature of the screen, black cloth, and ambient air; and 3) wind velocity. The convective heat transfer coefficients were determined as functions of the air–screen temperature difference under open-air environmental conditions. It was observed from the outcomes that the heat transfer coefficients decreased with the increase of the air–screen temperature difference provided that the wind velocity was nearly zero.

Film cooling performance measurement over a flat plate for a single row of holes embedded in an inclined trench

Film cooling heat transfer coefficient and effectiveness measurements downstream of a single row of holes embedded in an inclined trench injecting secondary fluid into the mainstream over a flat surface are reported here. The well-studied trench geometry is one where the walls of the trench are perpendicular and film injection holes are at an angle to the mainstream flow direction. In the current investigation, the trench is inclined at an angle to the mainstream and the injection holes are normal to the flow direction. Coolant injection in the form of a uniform stream at an angle to the mainstream is likely with this configuration. The trench inclination is kept constant at 35° and three hole pitch to diameter ratios equal to 3, 4.5 and 6 are studied for the blowing ratio varying between 0.3 and 2.5. The injection hole length to diameter ratios equal to 1.8 and 5.5 were studied and the influence on the net heat flux reduction ratio was found to be very small. The local effectiveness and the heat transfer coefficient values are presented and the variations in the cross-stream direction are observed to be very small. The effectiveness values for the current angular trench are higher whereas the heat transfer coefficients are smaller compared to the normal trench at higher blowing ratios. This results in increased Net Heat Flux Reduction ratio (NHFR) values for the current configuration which are observed to remain high at large downstream locations also.