Average Convective Heat Transfer Coefficient Research Articles

Research on temperature field model of hot air reflow soldering process

In this paper, two typical temperature models on the hot air reflow soldering process are studied. The critical difference between the two model is the approach used to compute the convective heat trans coefficient during the soldering process. In the model 1, the Martin formula and k-epsilon (2eqn) equation is applied to obtain the average convective heat transfer coefficient. And in model 2, the liquid and solid coupling CFD model is built to compute the convective heat transfer coefficient inherently. The comparing between the simulation and the test is conducted and shows that the simulation result of model 2 are closer to the real value. The comparing between these two models shows that the gas temperature, the gas velocity and the convective heat transfer coefficient around the PCBA is quite different. It is indicated that the average convective heat transfer coefficient calculated by Martin formula was larger. The CFD model is prefer for simulating the temperature distribution of the PCBA during the air reflow soldering when higher result precision is needed.

Distribution of heat transfer coefficient in the vertical tube of falling film evaporator treating saline wastewater based on micro flow and experimental verification

Abstract It is still one of the significant solutions to treat saline wastewater with thermal desalination technology, especially falling film evaporators. To improve the performance of the falling film evaporator, a numerical study on the gas–liquid two-phase flow characteristics of saline wastewater in the vertical pipe was conducted using the VOF model. The results showed that the inlet velocity of the saline wastewater increased under the same operating conditions, resulting in the thickening of the liquid film and the increase of the average convective heat transfer coefficient. Increasing the inlet temperature of the working liquid reduced the temperature difference, which led to a decrease of the average convective heat transfer coefficient. In addition, as the inlet concentration of the working liquid increased, the film flow rate and the average convective heat transfer coefficient first decreased and then increased slightly. The experimental results verified the accuracy of the numerical simulation, and the average error was 9.27%.

Thermal analysis for fundamental power coupler of HALF 499.8 MHz superconducting cavity

The preliminary research work of Hefei Advanced Light Facility (HALF), a fourth generation diffraction limit storage ring, is in progress. Two 499.8 MHz superconducting cavities may be used in the HALF RF system. RF power will be feed into each cavity through a coaxial fundamental power coupler (FPC). Thermal design of the coupler is very important because of the large power losses on the coupler will cause overheating. Accurate and fast thermal analysis is a challenge in the thermal design. In this paper, two thermal analysis methods for evaluating the cooling effect of the coupler were compared. Computational Fluid Dynamics (CFD) code is used to perform thermal-fluid coupled analysis for the coupler in the first method, which is quite accurate but time-consuming. In the second method, average convective heat transfer coefficient calculated by correlation equations based on experimental results is used to evaluate the heat transfer between the fluid and the cooling pipe wall. The calculation time of the first method is about four times that of the second method. The simulation results of the two methods are consistent well at high flow rate, especially for turbulent flow. Therefore, the second method was used to perform thermal simulation under different cooling conditions and RF power levels. The influence of thermal radiation on the heat load to the cryogenic environment was also given in this paper. Simulation results show that thermal radiation has a slight impact on the heat load. When the copper emissivity equal to 0.02, the total static and dynamic heat load to 4.5 K are 0.833 W and 2.614 W at the power level of 200 kW, and the heat load to 4.5 K cryogenic environment caused by thermal radiation is 0.065 W.

Effect of Flow Bypass on the Performance of a Shrouded Longitudinal Fin Array.(Dept.M)

Theoretical and experimental studies were carried out 10 investigate the effects of duct velocity, sin density and tip-to-shroud clearance on the flow bypass and its impact on the pressure drop across a longitudinal aluminum fin array and its thermal performance. The clearance was varied parametrically, stating with the fully shrouded case and variations of the channel height giving partially shrouded configuration of different clearance ratios were also carried out. The slow bypass was found to increase with increasing sin density and insensitive to the air slow rate. That effect of fin density decreased as the clearance increased. The calculated total pressure was greatly affected by fin density. For fully-shrouded sin array, with H/S equals to 8 and 12.72, the pressure drop increased by a factor of 4.3 and 20 of that with Hf/S equals to 3.4, respectively. The total pressure drop and the average convective heat transfer coefficients corresponding to the fully and partially shrouded fin array of Hf /S=3.4 were compared. Going from fully to partially shrouded one of the largest clearance ratio (C/Hf =0.89), the total pressure drop reduced by about 50%. For clearance ratios equal to 0.36, 0.56, and 0.89, the average heal transfer coefficients were reduced by about 12, 17, and 30 percent of those for the fully shrouded configuration at ReD of about 3x103. That percentage reduction in heat transfer coefficients decreased with the increase of air flow rate.

Flow and heat transfer characteristics of liquid metal sodium in tubes with different inner wall structures

In order to explore the influence of heat transfer medium on the enhancement of flow and heat transfer in the channel, the radial distribution of the streamwise velocity and temperature of water and liquid metal sodium in the square rib tube and the average convective heat transfer coefficient are compared. Numerical results show that the velocity gradient of liquid metal sodium near the wall is slightly larger than that of water, while the temperature gradient is significantly smaller. The heat transfer effect of liquid metal sodium is significantly better than that of water. When the velocity is 3m/s and 11m/s, the average convective heat transfer coefficient of liquid metal sodium increases by 220.59% and 193.42%, respectively, compared with water. In order to analyze the effect of inner strengthened structure on the flow and heat transfer characteristics of liquid metal sodium, the circular tube, square rib tube and circular arc rib tube are studied. Numerical simulation results show that both the average Nusselt number and pressure drop of square rib tube are the largest. When Peclet number is 270 and 990, the average Nusselt number of square rib tube increases by 13.60% and 57.89%, respectively, compared with that of circular tube.

Overall numerical simulation and experimental study of a hybrid oblique-rib and submerged jet impingement/microchannel heat sink

In this work, two submerged jet impingement/microchannel heat sink (JIMHS) models were proposed, i.e., straight-rib jet impingement/microchannel heat sink (SJIMHS) and oblique-rib jet impingement/microchannel heat sink (OJIMHS). The heat transfer and flow characteristics of the two models were investigated by overall numerical simulation and experiment. In the numerical simulation, the effects of heat flux, pressure drop and rib arrangement on the internal flow and heat transfer of the heat sink were studied. The results indicate that under the same heat flux and inlet condition, the heat transfer surface of OJIMHS achieves more uniform and lower temperature distribution compared with that of SJIMHS, and the average convective heat transfer coefficient of the OJIMHS is obviously higher than that of SJIMHS in all calculation cases, with an increase of about 20%. In addition, the performance of OJIMHS was tested experimentally. The comparison indicates that the maximum relative errors of average temperature and heat transfer coefficient between simulation and experiment were less than 9%. When the volume flow rate is 0.5 L/min and the heat flux is 100 W/cm², the average temperature of the heat transfer surface is still lower than 60°C. Besides, the averaged heat transfer coefficient of 2.8W/(cm2·K) was achieved under the inlet fluid temperature of 283K and volume flow rate of 2.5 L/min in the experiment.

Effects of temperature-dependent property variations on the output capacity prediction of a deep coaxial borehole heat exchanger

This study aims to understand the effects of property variations on prediction discrepancies in the output capacity (i.e., output power) of deep coaxial borehole heat exchangers under different operational conditions. These properties include the specific heat and thermal conductivity of the water and the reservoir rock, as well as the density and dynamic viscosity of the fluid. Numerical simulation and grey relational analysis were combined to quantify the effects of the property variations on heat convection, heat conduction, and the heat exchanger’s predicted output capacity. The simulation results indicate that the average convective heat transfer coefficient of the water decreases, but the average thermal diffusivity of the rock increases with the extraction of geothermal energy. These changes further lead to an over-prediction of the output capacity. The grey relational analysis reveals that the flow rate and well depth are the principal factors causing over-prediction. The over-prediction increases with the rise of the injection flow rate and well depth. The results also show that the variability of the specific heat and the dynamic viscosity of the water are the two most significant factors causing a discrepancy in the predicted output capacity. The results from this research have highlighted the significance of considering property variations for a more practical prediction of the numerically modeled output capacity of deep coaxial borehole heat exchangers.

MEAN NUSSELT NUMBER CORRELATION FOR TISE HEATSINK THERMAL DESIGN

This work was developed from a review of the technical literature for the thermal design of parallel plates heat sinks with uniform cross section cooled by airflow with the TISE (Top Inlet Side Exit) configuration. Due to an observed lack of agreement of the literature correlations among themselves and also with the available experimental results, numerical simulations were then performed to evaluate the forced convective heat transfer in the channels of these heat sinks with the TISE configuration. The simulations encompassed a range of heatsink airflow rates, considering distinct channel sizes and also the effect of a partial opening for the airflow entrance at the heat sink top. The obtained numerical results were employed to evaluate the average convective heat transfer coefficient inside the heatsink’s channels, based on the flow mixed mean temperature. A new empirical correlation was then proposed for the average Nusselt number as a function of the airflow Reynolds number and three dimensionless channel geometric parameters. The new correlation was compared with available experimental data.

Influence of thermodynamic disfigurement on the convective heat transfer of solar greenhouse

Solar greenhouse is a typical greenhouse without any additional heating system, which has developed rapidly in Northern China. However, due to the construction quality, management methods, especially the long-term use and other factors, there are usually different degrees of thermodynamic disfigurements in the envelop enclosure of solar greenhouse. The purpose of this study was to investigate the influences of thermodynamic disfigurement on the temperature distribution and convective heat transfer of solar greenhouse. In this study, the east and west compartments of a typical solar greenhouse which is located in Yangling, China (108°4′E, 34°16′N) were tested. The air temperature of each compartment was collected using temperature recorders, and the thermal infrared images of different compartment envelopes were obtained by a thermal infrared imager on a typical cloudy day. Convective heat transfer coefficients and heat flux densities of different compartment envelopes in the solar greenhouse were calculated. The results showed that the temperature difference can be displayed in the thermal infrared images of compartment envelopes, the surface temperature of the front roof was the lowest, followed by the back roof, the wall surface temperature was the highest. The minimum average surface temperature of the front roof in the eastern compartment was only 3.8°C, which was 6.8°C and 9.2°C lower than the average surface temperature of the back roof and back wall, respectively. The surface average temperature of thermodynamic disfigurements located at the bottom of the south side in the front roof of the eastern compartment, whose area accounted for 16.5% of the total front roof in the eastern compartment, was only 5.4°C. Compared with non-thermodynamic disfigurement, the average convective heat transfer coefficient and heat flux density of thermodynamic disfigurements in the front roof of the eastern compartment were increased by 20.3% and 110.3%, respectively. The average air temperature in the eastern compartment was 3.5°C lower than the average air temperature in the western compartment of the solar greenhouse. Construction of brick wall at the bottom of the south side of the front roof in the solar greenhouse helped to increase the inner surface temperature of the front roof, with an average temperature rise of 6.2°C, and reduce the area of thermodynamic disfigurement, which only accounted for 2.6% of the total front roof in the western compartment. The average surface temperature of thermodynamic disfigurements mainly caused by the entry and exit door in the wall of the eastern compartment was only 9.8°C, which was lower 3.2°C than the average temperature of non-thermodynamic disfigurement of the wall. Thermodynamic disfigurement helped to increase heat loss. The weighted average proportion of thermodynamic disfigurement in the western compartment was 2.1%, while that of thermodynamic disfigurement in the eastern compartment was 10.7%. The thermal insulation performance of the western compartment envelope in the solar greenhouse was better than that of the eastern compartment envelope. Keywords: solar greenhouse, thermodynamic disfigurement, convective heat transfer, thermal infrared image DOI: 10.25165/j.ijabe.20201304.5397 Citation: Sun Y C, Cao Y F, Wang H T, Zhou C J, Xiao J X, Jiang C Y, et al. Influence of thermodynamic disfigurement on the convective heat transfer of solar greenhouse. Int J Agric & Biol Eng, 2020; 13(4): 55–60.

Feasibility study of a new attached multi-loop CO2 heat pipe for bridge deck de-icing using geothermal energy

Heated bridge using CO2 heat pipe with geothermal energy as its heating source is a sustainable and environmentally friendly alternative to conventional chemical de-icers, which corrode the bridges and pollute the environment. However, in previous studies, the heat pipe was embedded in the bridge deck for heating and is only applicable to new bridges. In this study, a new attached multi-loop heat pipe system was developed to be used with ground heat exchangers for heating existing bridges. The heat pipe system, using CO2 as the heat transfer fluid, consists of an evaporator pipe and a manifold pipe with five pipe branches. A parametric study was first conducted on the system in a freezer box at controlled freezing temperatures to determine the optimum CO2 pressure and pipe inclination angle. Then the optimized heat pipe system was attached to the bottom surface of a concrete slab and covered with fiberglass and geofoam insulation. A water bath capable of supplying hot water similar to that from the ground heat exchanger was connected to the evaporator pipe as the heating source. The slab was heated at two freezing temperatures of −3.89 °C and −6.67 °C with the water bath temperature ranging from 25 to 45 °C. A 1-D heat transfer model using a finite difference method (FDM) was developed for the heat transfer analyses of the concrete slab. The average convective heat transfer coefficient (h value) of each test was back-calculated from the FDM model. Results show that an average 28.3% of the supplied heat energy was transferred to the slab surface, and the surface heat flux ranged from 92.8 to 140.3 W/m2. Compared with previous studies, the new CO2 heat pipe is feasible and efficient as a heating system attached to existing bridge decks for de-icing.

Numerical Studies on the Thermal Performances of Electroosmotic Flow in Y-Shaped Microchannel Heat Sink

This paper numerically studies the thermal performances of electroosmotic flow (EOF) in a symmetric Y-shaped microchannel heat sink (MCHS) having a constant total channel surface area, that is, constant convective heat transfer area. It is found that the average convective heat transfer coefficient of EOF increases with the increasing driven voltage, which is attributed to the increase of EOF flowrate with the increasing driven voltage. However, the maximum MCHS temperature shows an increasing after decreasing trend with the driven voltage owing to the dramatically increasing Joule heating when the voltage is large enough. Further, both the maximum MCHS temperature and average convective heat transfer coefficient are sensitive to the cross-sectional dimensions of the Y-shaped microchannels. The thermal performances of EOF in the Y-shaped MCHS show a strengthening to weakening trend with the increasing daughter-to-parent channel diameter ratio of the Y-shaped microchannel with circular cross-sectional shape, and show a similar strengthening to weakening trend with the increasing daughter-to-parent channel width ratio and the increasing microchannel height of the Y-shaped microchannel with rectangular cross-sectional shape. These cross-sectional dimension dependences of thermal performances are related to the increasing to decreasing trend of EOF flowrate changing with the microchannel cross-sectional dimensions.

Characteristics investigation on heat transfer growth of sonochemically synthesized ZnO-DW based nanofluids inside square heat exchanger

In recent decades, the growth of heat transfer using nanomaterials in the conventional base fluid has caught the attention of researchers around the world. The present research investigates the growth in heat transfer using ZnO-DW based nanofluids in a square heat exchanger. ZnO nanoparticles were synthesized by using the single-pot sonochemical technique. The ZnO-DW based nanofluids with different concentrations (0.1, 0.075, 0.05 and 0.025 mass%) were prepared by using probe sonication technique. The heat transfer growth will be benchmarked using the experimental data from distilled water experiment. Reynolds numbers, average convective heat transfer coefficient (h), and Nusselt number were calculated and analyzed in this investigation. Significant enhancement of 52% in thermal conductivity was noticed at 45 °C for 0.1 mass% concentration of ZnO-DW based nanofluids, which is due to the presence of maximum ZnO nanoparticles. Moreover, the maximum improvement in Nusselt values recorded at the end of the square pipe is 47% for 0.1 mass%, while 32%, 27% and 17% increase was recorded for 0.075, 0.05 and 0.025 mass% concentrations, and heat transfer enhancement was from 500 to 1100, 500 to 960, 500 to 910, and 500 to 900 W m−2 K−1 for different ZnO-DW based nanofluids mass% concentrations, which is more than water due to stability of nanoparticles. It can be concluded that the heat transfer performance enhancement is credited to the combination of square pipe and well-dispersed and stable ZnO-DW based nanofluids for heat exchanger applications.

Thermal performance of a ground U-shaped tube with twisted tapes in sand/graphite backfill materials

ABSTRACT Thermal characteristics of water in a ground U-tube with twisted tape inserts were addressed. At inlet temperatures of 60°C to 75°C, and Reynolds numbers of 3000 to 9000, the surface heat transfer coefficients for water with twisted tapes ranged from 442 to 897 W m−2 K−1, which were higher than that in smooth tube, 384–810 W m−2 K−1. Average convective heat transfer coefficient for the smooth tube in sand/graphite blend ameliorated by 36% compared to that in sand. Average apparent thermal diffusivities of sand/graphite at hot end temperature of 70°C rose by about 17% compared to sand.

Mixed convection and radiation from an isothermal bladed structure

The total heat transfer from a finned or ‘bladed’ structure may be expected to rise from that of a flat geometry due to a larger area exposed to ambient air. However, this is not always the case because a trade-off can be found between convection and radiation as the bladed geometry and surface temperature are varied. Furthermore, a mixed convection regime adds complexity to the heat exchange between the surrounding air and the heated bladed structure, which is relevant to various applications such as cooling and solar thermal energy. In this study, mixed convection and radiation heat transfer were determined for a cuboid with isothermal blades protruding from one of its sides. A steady-state simulation based on a three-dimensional SST k–ω turbulence model was performed in OpenFOAM to estimate the average convection heat transfer coefficient as a function of structure orientation, convection regime and bladed geometry. A Monte Carlo ray tracing method was employed to calculate view factors for determining the radiation heat transfer coefficient. Wind tunnel experiments validated the combined numerical approach. An increasing pitch angle (starting from the vertical) gradually increased convection heat transfer until a maximum value at ~45° and then significantly decreased it, where both effects were due to a flow behaviour in which vortices between the blades formed (<45°) but then disappeared (>45°). The velocity and wall temperature of the mixed convection flow in between the dominantly natural and forced convection regimes were identified. In comparison to a flat geometry, the bladed structure increased the temperature value at which the heat transfer by convection and radiation equalised, and produced a lower total heat transfer coefficient than the flat configuration at high surface temperatures. Furthermore, flow bifurcations were observed between the blades as their length was increased. In contrast, a large number of blades led to a flow transition where most of the incoming air exited from the side apertures to a lid-driven-like convection with flow recirculation occurring within the blades, which created a thermal barrier that decreased convection heat transfer. These results support the design of bladed or finned structures that enhance or reduce heat transfer by mixed convection and/or thermal radiation.

The influence of exterior louver blinds’ geometric and thermal attributes on the convective heat transfer at building facades

As an effective method to reduce indoor solar heat gain, louver blinds are increasingly utilized in buildings with high window-to-wall ratios. The blocking of these blinds changes the flow field around the building and then affect the convective heat loss of the façade behind. However, the present correlations utilized to describe the convective heat transfer at the surfaces of the façade and blinds are based on the studies focusing on a smooth plate or a smooth façade of a bluff body, whose accuracy has not been verified. Therefore, in this study, a wind-tunnel experiment is conducted using thermal balance analysis to investigate the convective heat exchange at the blinds and the façade behind. The results show that the current correlations will significantly underestimate the average convective heat transfer coefficient (CHTC) and Nusselt number (Nu) at the blinds while overestimating them at the building façade behind. The results also indicate that the convective heat transfer intensity at the blinds is positively correlated to φ (louver angle) and W (distance between the façade and blinds), whereas that at the façade behind is positively correlated to W, and negatively correlated to φ and θ (temperature difference between the façade and blinds). The natural convection around blinds in summer, which is characterized by the average RiB (Richardson number for the louver blinds), has little effect on the CHTC and Nu at the façade and blinds. This study provides a contribution to clarify the thermal performance of the building envelopes attached with louver blinds.

DETERMINATION OF CONVECTIVE, RADIATIVE, AND TOTAL HEAT TRANSFER CHARACTERISTICS OVER A RADIANT HEATED CEILING: A COMPUTATIONAL APPROACH

also been given in international standards for experimental approaches. In accordance with the experimental standards, the analyses are realized at the room dimensions of (L x H x W= 4 x 3 x 4 m), and at the thermal boundary conditions of (Tc= 35 – 50oC), (Tw= 15 – 24oC), (Tf= 15 – 24oC). As a result of iterations conducted using a computational fluid dynamics package program code, each corresponding to a case study, convective, radiative, and total heat fluxes from radiant heated ceiling are obtained, and afterward using relevant reference temperature proposed in the literature, heat transfer coefficients pertaining to convection, radiation, and total heat transfer have been found. In conclusion, average heat flux values of 95.83 Wm-2 and 17.49 Wm-2 are gained through the examined radiant heated ceiling, by means of radiation and convection, respectively. This denotes that roughly 85% of total heat transfer from the ceiling arises through radiation that is very compatible with the relevant literature. Finally, spanning the entire different case studies, average convective, radiative, and total heat transfer coefficients of 1.1 Wm-2K-1, 5.4 Wm-2K-1, 7.6 Wm-2K-1 are acquired.

Experimental Investigation of Convective Heat Transfer of Supercritical Pressure Hydrocarbon Fuel in a Horizontal Section of a Rotating U-Duct

Abstract The experimental and numerical investigations of the heat transfer of supercritical pressure n-decane flowing through a pipe at various rotational speeds, mass flow rates, heat fluxes, and pressures, are presented. This pipe is 2 mm in diameter, 200 mm in length, with a radius of 0.328 m, and is parallel to the rotating axis. The wall temperature was measured at four positions around the periphery of the pipe at each of the five selected cross section along the pipe's length. Maximum convective heat transfer was observed at the outer edge of the horizontal section, while its corresponding minimum was observed at the inner edge. The heat transfers at the two sides of the channel were observed to be similar. The density and pressure differences between the outer and inner edges increased at increasing rotating speeds. However, the temperature difference between the outer and inner edges decreased with increased rotational speed mainly because of the increase of secondary flows in the section. The section's average convective heat transfer coefficient increased with an increase in the rotational speed, and its value at 1000 rpm was approximately twice than that at static conditions. The phenomenon of oscillation was observed near the exit of the horizontal section, and was caused by the flow and considerable property changes near the pseudo critical temperature. A computational fluid dynamics (CFD) model was developed using the real gas thermal properties and was coupled with the heat transferred owing to fuel flow. The predicted fuel and wall temperatures were in good agreement with the experimental data. A new local Nusselt number correlation of the heat transfer of n-decane in a rotating horizontal section was proposed.

Experimental Study on LED Heat Dissipation Based on Enhanced Corona Wind by Graphene Decoration

To enhance the heat dissipation of high-power light-emitting diodes (LEDs), corona wind based on corona discharge was promoted. In this paper, we aimed at reinforcing the intensity of corona wind by modifying the needle electrode with graphene solution. To explore the differences in the graphene-coated electrode and the original bare electrode on the electrical characteristics, the intensity of corona wind, and LED heat dissipation, a corona wind cooling system and a thermal test system were built for the experimental study. The results showed that the graphene-coated electrode system could produce corona wind at a lower voltage. In addition, the discharge current of the graphene-coated electrode system is higher than that of the bare electrode system under the same discharge voltage. The graphene-coated electrode system can induce a faster corona wind and the highest speed is about 3.4 m/s, showing that it is more efficient than the bare electrode system in corona wind production. Moreover, the temperature drop of LEDs for the graphene-coated system is greater, which means that the corona wind cooling system can reduce the junction temperature of the LED chips effectively. Ultimately, as the discharge power increases, the average convective heat transfer coefficient increases. Indeed, the result of this paper can be used for the wide variety of the graphene-coated corona wind generator application as a promising technology for the LED heat management.

Influence of thermal boundary conditions on local convective heat transfer in coiled tubes

Helical coil heat exchangers are commonly used in a wide variety of engineering applications because, due to curvature of the tube that generates secondary flows, this kind of geometry can generate an enhancement of the convective heat transfer coefficient. Since the distributions of the fluid velocity and temperature in coiled pipes are strongly asymmetrical over the cross-section of the tube, particularly in laminar flow conditions, it is important to monitor the local heat transfer phenomena, mainly with regard to the applications where the local behaviour plays a leading role, such as in food industry. Indeed, due to the high viscosity of fluid foods or to the low velocity in which they may operate, the flow regime is often laminar in this kind of devices. In laminar flow regime, the thermal boundary conditions considerably affect the convective heat transfer performance. In the present paper, a numerical investigation focused on the evaluation of the effect of the choice of the thermal boundary condition on the local heat transfer phenomena in coiled-tubes, within the laminar regime, is presented. To cover realistic configurations in which the helical coil heat exchangers are commonly used, the conjugate heat transfer problem in the tube was also considered. The momentum and energy equations were solved by means of the open-source CFD software OpenFOAM®, with second-order accurate finite-volume schemes. The here adopted numerical model was validated by comparing the results with local experimental data. The numerical outcomes, obtained for several values of the Prandtl number, revealed that the thermal boundary conditions could significantly change the local distribution of the convective heat transfer coefficient. Finally, the influence of the thermal boundary condition on the average convective heat transfer coefficient were evaluated by considering the two definitions of the circumferentially averaged Nusselt number commonly used in literature.

EXPERIMENTAL DETERMINATION OF CONVECTIVE HEAT TRANSFER COEFFICIENTS IN THERMAL BUOYACY VENTILATION SYSTEM

Objectives. The main goal of the article is to present the developed method for the experimental determination of convective heat transfer coefficients, suitable for studying the internal convection of models of complex configuration. Method. The study of free convection under the conditions of an internal problem was carried out by determining the conditional thickness of the boundary layer by a graphic method. The first was the selection of the calculated sections and planes for the experimental installation. The selection is carried out in such a way that the calculated planes are perpendicular to the heated walls of the channel in question. Installation of an experimental model is possible only in a room with low internal air mobility, as well as a stable temperature. In this room there should not be heating and heating devices that can create strong convective currents near the channel of the experimental installation. Result. The article presents the results of an experimental study to determine the temperature distribution of the air flow and average convective heat transfer coefficients over the height of the ventilation channel. A decrease in convective heat transfer coefficients at an altitude of 0.5 to 1 meter occurs less noticeably than at an altitude of 1 to 2 meters, which is associated with the restoration of flow after a vent removal. At the stabilization section, there is first a gradual decrease, and then an increase in axial velocity, which is caused by the merging of multidirectional air flows in this area. Conclusion. It was revealed that in the case of modeling free convection under the conditions of an internal problem in the presence of heat-removing boundaries within the limits of the calculated temperature difference, taking into account the flow turbulization has practically no effect on the final results.