Heat Transfer Coefficient Data Research Articles

Field measurement and modeling of UVC cooling coil irradiation for heating, ventilating, and air conditioning energy use reduction (RP-1738)—Part 1: Field measurements

Wet cooling coil surfaces can provide opportunities for microorganism growth. This biological fouling (biofouling) increases airside pressure drop and decreases airside heat transfer coefficient. Ultraviolet germicidal irradiation is one method to eliminate biofouling. The current article reports field measurements of changes in pressure drop and heat-transfer characteristics of fouled coils treated with ultraviolet germicidal irradiation designed for surface disinfection. Ultraviolet germicidal irradiation systems were installed in operating air-handling units with visibly fouled cooling coils at sites in Tampa, Florida and University Park, Pennsylvania. Pressure drop data were controlled for airflow and latent load, and overall heat-transfer coefficient data were controlled for heat exchanger entering conditions. Analysis of Tampa site data showed an initial mean 22.70% to 22.72% decrease (µ ± 2σ) in airside pressure drop and a mean 13.2% to 13.6% (µ ± 2σ) increase in overall heat-transfer coefficient. Longer-term improvement was 14.6% for the airside pressure drop and 15.7% for overall heat-transfer coefficient, though the decrease in pressure drop improvement was not able to be explained. The University Park, Pennsylvania site, cleaned by unaware maintenance personnel before data collection, had an initial mean 10.3% pressure drop improvement and 57.7% overall heat-transfer coefficient improvement, followed by a long-term 0% pressure drop improvement and 36.7% overall heat-transfer coefficient improvement.

Flow boiling of R32 in a horizontal smooth tube of 6.0 mm internal diameter: heat transfer coefficient and pressure drop

This paper presents a series of experiments on flow boiling heat transfer coefficient and pressure drop using refrigerant R32. Tests were performed in a smooth horizontal stainless steel tube with an internal diameter of 6.00 mm, with the heat directly applied on the test section tube through DC current. Trends of the heat transfer coefficient and pressure drop data are displayed for different operating conditions. Specifically, the saturation temperature was fixed to 25 and 35 °C, the heat flux varied from 5 to 50 kW/m2 and the mass velocity was fixed to the low values of 150 and 250 kg/m2 s. The effects of the operating parameters on the heat transfer results are also shown.

Heat transfer distribution in helical coil flow boiling system

The objective of the present work is to study the heat transfer distribution in the helical coil flow boiling system. Infrared thermal imaging technique is used to measure the local wall temperature. The wall temperature is measured across the circumference and in axial direction along the length of a helical coil. The literature showing circumferential heat transfer distribution in a helical coil during flow boiling process is scarce. The present work compares the wall temperature and heat transfer coefficient distribution on the inner side and the outer side of helical coils at different diameter ratios and density ratios. In the present study, the experimental databank (containing total 400 heat transfer coefficient data points) includes the data not only from the present experimental study but also from the literature. The data bank includes the subcooled and the saturated flow boiling data. The data includes two fluids namely water and R123. Data covers the range of parameters namely a density ratio of 30–1600, a mass flux of 100–1300 kg/m2 s, a heat flux of 2–640 kW/m2, an exit quality of −0.5 to 1 and a coil to tube diameter ratio of 14–58. Ten empirical correlations which include well referred correlations of straight tubes and available correlations of helical coils are evaluated with the databank. The study concluded that the circumferential wall temperature variation in helical coils decreases with increase in a diameter ratio and with decrease in a density ratio. The circumferential averaged heat transfer coefficient during a saturated flow boiling in a helical coil is same as a straight tube. A correlation is suggested to measure the circumferential average heat transfer coefficient in helical coils.

A-19 シーリングファンを用いたオフィス空間内の熱環境調整に関する研究 : (その4)サーマルマネキンによる人体各部位対流熱伝達率の測定および65MNへの適

A-19 シーリングファンを用いたオフィス空間内の熱環境調整に関する研究 : (その4)サーマルマネキンによる人体各部位対流熱伝達率の測定および65MNへの適

New method of processing heat treatment experiments with numerical simulation support

In this work, benefits of combining modern software for numerical simulations of welding processes with laboratory research was described. Proposed new method of processing heat treatment experiments leading to obtaining relevant input data for numerical simulations of heat treatment of large parts was presented. It is now possible, by using experiments on small tested samples, to simulate cooling conditions comparable with cooling of bigger parts. Results from this method of testing makes current boundary conditions during real cooling process more accurate, but also can be used for improvement of software databases and optimization of a computational models. The point is to precise the computation of temperature fields for large scale hardening parts based on new method of temperature dependence determination of the heat transfer coefficient into hardening media for the particular material, defined maximal thickness of processed part and cooling conditions. In the paper we will also present an example of the comparison standard and modified (according to newly suggested methodology) heat transfer coefficient data’s and theirs influence on the simulation results. It shows how even the small changes influence mainly on distribution of temperature, metallurgical phases, hardness and stresses distribution. By this experiment it is also possible to obtain not only input data and data enabling optimization of computational model but at the same time also verification data. The greatest advantage of described method is independence of used cooling media type.

An experimental study of forced convective heat transfer from smooth, solid spheres

Forced convective heat transfer from smooth, solid and isothermal spheres of various diameters has been studied experimentally in air flows with various free-stream velocities. The average heat transfer coefficient has been determined from the steady measured power input to a heating element inside the spheres and the steady measured temperatures of the flowing air and of the surface of the spheres, employing corrections to account for heat transfer due to thermal radiation and due to natural convection. The current data for the average heat transfer coefficient, expressed as a relationship between the Nusselt number and the Reynolds number, complement data in literature with respect to the range of large Reynolds numbers that have been considered: here the Reynolds numbers were between 7.8×103 and 3.3×105. The experimental results show a sudden increase in the Nusselt number above a critical Reynolds number of approximately 2.9×105, analogous to the “drag crisis” for the drag force on the sphere. A correlation for the Nusselt number as a function of the Reynolds number has been formulated for air flows that describes these data well for Reynolds numbers below the critical Reynolds number.

Experimental heat transfer and pressure drop in a metal-foam-filled tube heat exchanger

Heat transfer in small tubes is of particular interest in the heat exchanger industry for manufacturing shell-and-tube or concentric-tube heat exchangers. Metal-foam-filled tubes enhance the heat transfer mechanism by providing a high surface-area-to-volume ratio. This experimental study investigates the heat transfer and pressure drop in such tubes. Single-phase experiments were performed using copper tubes with an inner diameter of 4mm and filled with copper metal foam. R245fa refrigerant was used as the working fluid with mass flux ranging from 200 to 1000kg/m2s. The heat transfer coefficient and pressure drop data are reported and compared to a tube without metal foam. The experimental data are also compared to well-known correlations from the literature. Most of the correlations are unable to capture any data point since they were developed for much bigger channels. New correlations are proposed to predict the heat transfer coefficient and pressure drop in such small tubes with metal foam inside.

Experimental Investigation of Local and Average Heat Transfer Coefficients Under an Inline Impinging Jet Array, Including Jets With Low Impingement Distance and Inclined Angle

Comprehensive impingement heat transfer coefficients data are presented with varied Reynolds number, hole spacing, jet-to-target distance, and hole inclination utilizing transient liquid crystal. The impingement configurations include: streamwise and spanwise jet-to-jet spacing (X/D, Y/D) are 4∼8 and jet-to-target plate distance (Z/D) is 0.75∼3, which composed a test matrix of 36 different geometries. The Reynolds numbers vary between 5,000 and 25,000. Additionally, hole inclination pointing to the upstream direction (θ: 0 deg∼40 deg) is also investigated to compare with normal impingement jets. Local and averaged heat transfer coefficients data are presented to illustrate that (1) surface Nusselt numbers increase with streamwise development for low impingement distance, while decrease for large impingement distance. The increase or decrease variations are also influenced by Reynolds number, streamwise and spanwise spacings. (2) Nusselt numbers of impingement jets with inclined angle are similar to those of normal impingement jets. Due to the increase or decrease variations corresponding to small or large impingement distance, a two-regime-based correlation, based on that of Florschuetz et al., is developed to predict row-averaged Nusselt number. The new correlation is capable to cover low Z/D∼0.75 and presents better prediction of row-averaged Nusselt number, which proves to be an effective impingement design tool.

Evaluation of thermal-hydraulic performance of hydrocarbon refrigerants during flow boiling in a microchannels array heat sink

The present study concerns an experimental evaluation of the performance of hydrocarbon refrigerants during flow boiling in a microchannels array heat sink. The heat sink is composed of fifty channels with cross sectional areas of 123×494μm2 and length of 15mm manufactured in a copper block. Heat transfer coefficient and pressure drop data were obtained for refrigerants R600a, R290 and R1270, mass velocities from 165 to 823kg/m2s, heat fluxes up to 400kW/m2, liquid subcooling at the inlet of the test section of 5, 10 and 15°C and saturation temperature of 25°C. The data were compared with experimental results obtained in a previous study for R134a and predictions by methods from literature. In general, R290 presented the best performance, providing the highest average heat transfer coefficient and a pressure drop only slightly higher than R1270 that was the fluid presenting the lowest pressure drop. An exergy analysis also revealed the refrigerant R290 as the one presenting the best performance. However, R290 needed the highest excess of superheating to trigger the boiling process (ONB). The methods from literature evaluated in the present study poorly predicted the experimental data for two-phase pressure drop. On the other hand, the method of Kanizawa et al. (2016) was quite accurate in predicting the heat transfer results.

Effect of Endwall Contouring on a Transonic Turbine Blade Passage: Heat Transfer Performance

Effect of turbine endwall contouring on its aerodynamic performance has been widely studied, but only a few studies are available in the open literature investigating its effect on heat transfer performance; especially at transonic exit Mach number conditions. In this paper, we report a study of effect of contouring on endwall heat transfer performance of a high-turning high-pressure (HP) turbine blade passage operating under transonic exit conditions. The paper describes comparison of heat transfer performance of two contoured endwall geometries, one aerodynamically optimized (AO) and the other heat transfer optimized (HTO), with a baseline, noncontoured geometry. The endwall geometries were experimentally investigated at Virginia Tech's transient, blow down, transonic linear cascade facility at three exit Mach numbers, Mex= 0.71, 0.88(design) and 0.95, for their heat transfer performance. Endwall surface temperatures were measured using infrared (IR) thermography and local heat transfer coefficient (HTC) values were calculated using measured temperatures. A camera matrix model-based data postprocessing technique was developed to relate the two-dimensional images captured by IR camera to three-dimensional endwall contours. The measurement technique and the methodology for postprocessing of the heat transfer coefficient data have been presented in detail. Discussion and interpretation of experimental results have been augmented using aerodynamic CFD simulations of the geometries. Both the contoured endwalls demonstrated a significant reduction in the overall average heat transfer coefficient values of the order of 10%. The surface Stanton number distributions also indicated a reduction in the level of hot spots for most of the endwall surface. However, at some locations an increase was also observed, especially in the area near the leading edge (LE). The results indicate that the endwall contouring could significantly improve heat transfer performance of turbine passages.

Experimental study on heat transfer characteristics of LN2 saturated flow boiling in a horizontal corrugated tube

The two-phase heat transfer of saturated liquid nitrogen (LN2) flow boiling in a horizontal corrugated tube was experimentally studied. The experiments were conducted at different inlet pressures of 0.180–0.461MPa and mass fluxes of 163.1–257.1kgm−2s−1 under a wide range of heat flux of 16,000–40,000Wm−2. The flow pattern was identified to be unstratified flow according to the circumferential distributions of heat transfer temperature differences. Two heat transfer regimes were observed in the present experiment: the nucleate boiling dominated heat transfer and the convective evaporation dominated heat transfer. Due to the thinning of the liquid film at the tube inner surface caused by helical fin, nucleation suppression and convection augmentation were observed and the variation of the circumferential average HTC with the flow parameters became complex depending on the relative importance of the two heat transfer mechanisms. The effects of heat flux, vapor quality, inlet pressure and mass flux on the circumferential average heat transfer coefficient (HTC) are discussed. The results show that the nucleate boiling heat transfer is enhanced with the increase of heat flux, while the convective evaporation is enhanced with the increase of mass flux and vapor quality, and the decrease of pressure. In order to improve the prediction of HTCs, the Klimenko correlation for cryogenic fluids are included in an additive model. Based on the experimental data of circumferential average heat transfer coefficients, the parameterization of the additive model including the convective and nucleate boiling components was obtained for the LN2 saturated flow boiling in the corrugated tube. Compared to the Klimenko correlation for the flow boiling of cryogenic fluids with a fully wetted perimeter in smooth tubes, the proposed parameterization of the additive model improves the prediction of HTCs in the convective evaporation dominated regime.

Experimental investigation of non-boiling gas-liquid two phase flow in upward inclined pipes

In comparison to horizontal and vertical two phase flow, very little information is available on the two phase flow phenomenon in upward inclined pipes. To explore and contribute to the current state of knowledge, the main objective of this work is to experimentally investigate the effect of upward pipe inclinations (horizontal to vertical upward) on gas-liquid two phase flow phenomenon. To accomplish this objective, experiments focused on flow visualization, void fraction, pressure drop and heat transfer measurements in non-boiling gas-liquid two phase flow are carried out in 12.7mm I.D. polycarbonate and 12.5mm I.D. stainless steel pipes using air-water as fluid combination. Different flow patterns are generated by varying the gas and liquid phase mass flow rates in a range of 0.001–0.2kg/min and 1–10kg/min, respectively. Experimental data for void fraction, pressure drop and heat transfer coefficient is analyzed for its dependency on phase flow rates and pipe inclination. Two distinct trends of the relation between these two phase flow variables and the phase flow rates or alternatively the flow patterns are observed. The experimental results show that increase in pipe inclination from horizontal significantly affects the void fraction, total pressure drop and heat transfer coefficient at low values of gas and liquid flow rates. With increase in the gas and liquid flow rates, effect of pipe inclination on two phase flow variables is observed to diminish. The issue of decreasing pressure gradient minimum in upward pipe inclinations is analyzed using non-dimensional form of gas and liquid flow rates. Finally, based on the heat transfer measurements, the circumferential variation of two phase heat transfer coefficient and its relation to the flow symmetry is discussed.

Experimental investigation of saturated flow boiling heat transfer of nitrogen in a macro-tube

The saturated flow boiling heat transfer of nitrogen (N2) in a vertical upward 11.9mm inner diameter stainless steel tube was experimentally investigated. The heat transfer coefficient (HTC) was measured as a function of vapor quality. Totally 414 experimental data points were obtained, with the parameter range of vapor quality from 0.01 to 0.99, pressure from 0.84 to 2.29MPa, heat flux from 9 to 98.8 kW/m2, and mass flux from 110 to 800kg/m2s. The effects of vapor quality, heat flux, mass flux, and pressure on the HTC are explored. Four flow patterns are identified by analyzing the HTC data, including bubbly flow, annular flow with liquid in the core and vapor between the liquid and the tube wall, dispersed flow, and mist flow. The trend of the HTC along the tube length (with vapor quality) is explained in relation to flow patterns. The flow and heat transfer characteristics in the experiments are compared with those in horizontal macro-tubes. Substantial differences between these two flow directions are revealed. For upward flow in vertical macro-tubes, it is fairly commonly seen that there are two occurrences of critical heat flux (CHF) along a uniformly heated channel. The mechanisms resulting in this phenomenon are found. The experimental data are compared with the correlations of saturated flow boiling heat transfer coefficient. The best one has a mean absolute deviation of 16.6%.

Effect of gravity vector on flow boiling heat transfer, flow pattern map, and pressure drop of R245fa refrigerant in mini tubes

Organic Rankine cycles have been proposed to address the on-going issue of global warming by recovering waste heat. R245fa refrigerant has high performance as a working fluid in such cycles. This study looks at the flow boiling and heat transfer mechanism of this refrigerant in a 3-mm stainless-steel tube. Horizontal flow and vertical flow are both considered. The flow pattern in mini tubes is visualized, and flow pattern maps are presented. Annular flow, slug flow, and bubbly flow are present in the horizontal flow. The vertical flow shows annular flow, churn flow, slug flow, and bubbly flow. Heat transfer coefficient data are also given for a mass flux range of 200–700kg/m2s, heat flux of 10–40kW/m2, and inlet quality of 0.1–0.7. The pressure drop and heat transfer mechanism are discussed. The horizontal flow data were compared with the vertical upward flow data. The experimental data for heat transfer coefficient and pressure drop was compared to the well-known correlations from the literature. The correlation by Liu & Winterton for heat transfer coefficient and the homogenous equation of Cicchitti et al. for pressure drop was found to wok the best for this range of experimental conditions. Also the flow patterns were well-predicted by using Ong and Thome correlation. The flow pattern base model of Cioncolini and Thome was able to capture most of the heat transfer coefficient and pressure drop data in annular flow. The results can be used for designing evaporators to consider the differences between vertical and horizontal flow regarding phase-change characteristics in heat exchangers orientation.

Flow boiling visualization and heat transfer in metal-foam-filled mini tubes – Part II: Developing predictive methods for heat transfer coefficient and pressure drop

High-porosity metal foams are proposed for insertion into heat exchanger channels to enhance the heat transfer mechanism in evaporators that are mainly used in organic Rankine cycle applications. This two-part experimental study investigates the two-phase flow inside a circular copper mini tube. Part I visualized the flow pattern and presented flow pattern maps, heat transfer coefficient data, and pressure drop data. Part II compares experimental data to recent correlations developed for metal-foam-filled mini tubes. The experimental conditions of the present study fall outside the valid range of the correlations, so none of them can predict the heat transfer coefficient or pressure drop accurately. Therefore, both the heat transfer coefficient and pressure drop are correlated using new approaches based on previous works and the present experimental data. Good agreement is observed between the experimental data and the new correlations in these experimental conditions.

Measured and predicted heat transfer coefficients for boiling zeotropic mixed refrigerants in horizontal tubes

The use of mixed gas working fluids has become common in Joule–Thomson type cryocoolers for a variety of applications. However, there is a scarcity of data currently available with supporting theory capable of predicting the heat transfer coefficients associated with two-phase, multi-component mixtures at cryogenic temperatures. This paper aims to fill this void by providing experimental data for the heat transfer coefficient associated with of multicomponent zeotropic mixtures boiling in small channels over temperatures ranging from 100K to room temperature. The sensitivity of the measured heat transfer coefficient to parameters such as heat flux, mass flux, pressure, tube diameter, and mixture composition is also presented. The results indicate that the heat transfer process is driven, principally, by convective boiling; however, composition, diameter, and surface roughness affect the measured heat transfer coefficient. Evaporating pressure has less relevance compared to the other parameters.The actual experimental data collected as part of this effort and additional data from Nellis et al. (2005) are used to evaluate models to characterize the heat transfer process for boiling zeotropic mixtures in horizontal tubes. The heat transfer coefficient data is predicted well using correlations described by Granryd (1991) and Little (2008). The Granryd correlation is the recommended correlation to predict heat transfer coefficient of zeotropic mixtures because it shows the best accuracy with an Absolute Average Deviation (AAD) of 16% and predicts 83% of the data with a relative error lower than 25%. The Little correlation exhibits greater accuracy at high Reynolds number, high thermodynamic quality, or both; however, its accuracy is substantially reduced for low qualities and low Reynolds number.

The Relative Performance of External Casing Impingement Cooling Arrangements for Thermal Control of Blade Tip Clearance

As a key way of improving jet engine performance, a thermal tip clearance control system provides a robust means of manipulating the closure between the casing and the rotating blade tips, reducing undesirable tip leakage flows. This may be achieved using an impingement cooling scheme on the external casing. Such systems can be optimized to increase the contraction capability for a given casing cooling flow. Typically, this is achieved by changing the cooled area and local casing features, such as the external flanges or the external cooling geometry. This paper reports the effectiveness of a range of impingement cooling arrangements in typical engine casing closure system. The effects of jet-to-jet pitch, number of jets, and inline and staggered alignment of jets on an engine representative casing geometry are assessed through comparison of the convective heat transfer coefficient distributions as well as the thermal closure at the point of the casing liner attachment. The investigation is primarily numerical, however, a baseline case has been validated experimentally in tests using a transient liquid crystal technique. Steady numerical simulations using the realizable k–ε, k–ω SST, and EARSM turbulence models were conducted to understand the variation in the predicted local heat transfer coefficient distribution. A constant mass flow rate was used as a constraint at each engine condition, approximately corresponding to a constant feed pressure when the manifold exit area is constant. Sets of local heat transfer coefficient data generated using a consistent modeling approach were then used to create reduced order distributions of the local cooling. These were used in a thermomechanical model to predict the casing closure at engine representative operating conditions.

Flow boiling characteristics of R134a and R245fa mixtures in a vertical circular tube

Considerable attention has recently been given to making new refrigerant mixtures for applications such as organic Rankine cycles (ORCs). In this study, the flow boiling of a mixture of R134a and R25fa was visualized. Mixtures at different molar fractions ranging from pure R245fa to a molar concentration up to 50% R245fa–50% R134a were chosen. The ranges of mass flux and heat flux were 300–800kg/m2s and 1–69kW/m2, respectively. A circular glass tube with a 3-mm inner diameter and 200-mm length was used. In each case, different flow regimes were recognized, such as bubbly flow, slug flow, and annular flow. Throat-annular flow was observed in limited cases but not in all mixtures. The heat transfer coefficient in each case was analyzed carefully. Degradation of the heat transfer coefficient due to the mass transfer resistance is discussed, and the heat transfer coefficient of the mixtures is shown to be lower than that of the original pure refrigerants. The heat transfer coefficient data were compared to predictive methods in the literature, and a modified version of correlations is proposed for the mixture of R134a and R245fa.

Heat transfer and pressure drop measurements in a square cross-section converging channel with V and W rib turbulators

Ribs on the opposite walls of internal cooling passages of gas turbine blades are often used for heat transfer enhancement. These passages can be straight, converging or diverging. In the present study, experimental data for local heat transfer coefficients are presented for a converging channel with rib roughening elements with the cross-section being maintained square from inlet to exit. The local heat transfer coefficient distribution shows the same qualitative behavior observed for the straight channel. The overall averaged heat transfer coefficient along with the measured pressure drop across the test channel is used to calculate thermal performance based on constant pumping power and constant heat transfer area criterion.Data are presented for straight ribs normal to the flow, V ribs with included apex angle of 45° and with apex pointing in the upstream and downstream directions, W ribs with all included angles of 45° and with central apex pointing in the upstream and downstream directions. The inlet Reynolds number (Rein) was varied from 5000 to 35,000. The rib height (e) was maintained constant throughout the length of the channel. The rib height to mean duct hydraulic diameter ratio (e/Dh,m) was kept constant at 0.08. Data for three pitch to height ratios (P/e) equal to 6, 10 and 17.5 are reported for straight and V ribs. The optimum P/e ratio based on constant pumping power thermal performance criterion was observed to be equal to 10 and this was the only configuration studied for W ribs. The difference in thermal performance between the V and W ribs was within uncertainty limits. However, the local variation in heat transfer coefficient in the cross stream direction on the ribbed wall was noticed to be very high for the V ribs compared to that for the W ribs making the W ribs a better choice as enhancement device.

Experimental investigation of flow boiling heat transfer and pressure drops characteristic of R1234ze(E), R600a, and a mixture of R1234ze(E)/R32 in a horizontal smooth tube

The saturated flow boiling heat transfer coefficients and frictional pressure drops characteristics of R1234ze(E), R600a, and L-41b (R1234ze(E)/R32 (27/73 mass %)) inside an 8 mm inner diameter horizontal tube were investigated. The experiment was carried out at the saturation temperature of 20°C with heat flux ranging from 5.0 to 10.0 kW·m−2 and mass flux ranging from 200 to 400 kg·m−2·s−1. The influence of mass flux, heat flux, and quality on the heat transfer coefficients and frictional pressure drops were examined and discussed. The results show that the local heat transfer coefficients of R1234ze(E) are averagely 33% and 18% lower than those of R600a and L-41b, respectively. The frictional pressure drops of R1234ze(E) are 21% lower than those of R600a but 6% greater than those of L-41b. Meanwhile, the experimental data of local heat transfer coefficients and frictional pressure drops are compared with some well-known correlations available in literatures.