Experimental Study Of Heat Transfer Research Articles

Experimental study and numerical modeling of heat and mass transfer in rubberwood during kiln drying

Rubberwood (Hevea brasiliensis) is a major raw material used in furniture factories in southern Thailand. Proper kiln drying of the lumber plays a significant role on the efficient utilization of the final product. Overall quality-reducing defects in such a materials are inevitable, unless a correct and proper drying process is used. Therefore, the objective of the present study was to develop a three-dimensional numerical model of heat and moisture transfer during the drying process and to conduct physical experiments in order to compare the results between the two approaches. For this purpose, an innovative kiln dryer was designed in the laboratory which will also contribute to the prevention of defects in the wood. The results show that the samples dried at a temperature of 120 °C experienced faster decreases in their moisture content compared to those dried at temperatures of 100 °C and 80 °C. The drying temperature and moisture content of the samples were successfully predicted by the simulation model created in this investigation and the two approaches demonstrated good agreement for temperature and moisture with root mean square errors of 0.50 and 0.19, standard errors of 0.38 and 0.16, and mean absolute errors of 0.94 and 2.18, respectively. These results will help rubberwood drying facilities prevent wood deformation while reducing the drying time required.

An experimental and numerical study of forced convection heat transfer from rectangular fins at low Reynolds numbers

This paper presents an experimental and numerical study of forced convection heat transfer from straight rectangular fins on a horizontal surface at low Reynolds numbers ranging from 2600 to 6800. Heat transfer for a fixed number of fins, fin spacing, and length was measured in a wind tunnel for varying inlet air velocity and heat input. Three-dimensional conjugate heat transfer simulations were also conducted to characterize important features of heat transfer and flow of the fin array and the effects of fin channel length on heat transfer. A maximum relative error of about 4.28% was found between the experimental and the numerical results for the average Nusselt number. The experimental results showed that forced convection heat transfer is characterized by an approximately linear relationship between the Nusselt and Reynolds numbers, which also agreed reasonably well with the available correlations for fully developed turbulent convective heat transfer for tubes with uniform heat flux. Further investigations of the effects of fin geometry were made numerically. Shorter fin channels, where the flow is thermally and hydrodynamically developing, were more effective in heat transfer than longer ones, where the flow approaches or reaches full development. It is shown that heat transfer rate per channel decreases linearly with the increase in channel length, but remains approximately constant with the increase in number of fins. It is also shown that the contributions of natural convection and radiation were negligible for the specific fin design and the range of inlet flow conditions and maximum fin temperature investigated here.

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 study of convective heat transfer during liquid piston compressions applied to near isothermal underwater compressed-air energy storage

Abstract Compressed air energy storage may be an important asset for large-scale implementation of renewable energies. The REMORA technology combines quasi-isothermal compression with maritime construction efforts. Convective heat transfer in liquid piston compressors have not been thoroughly studied so far, but its understanding is deeply important to characterize the system's behavior. An experimental analysis of air compression using liquid piston is proposed for chambers with big dimensions, which leads to the development of several correlations. From these assessments, it is possible to describe the presence of two different convective heat exchange patterns, as well as the liquid piston position in the transition between them. A numerical model is developed by employing these correlations. The comparison between numerical and experimental results enables the validation of such correlations for a specific set of parameters.

Experimental study of free convective heat transfer around a spherical electronic component cooled by means of porous media saturated by nanofluid

This experimental work deals with quantification of free convective heat transfer around a spherical electronic device. This so-called active sphere generates during its operation a heat flux leading to Rayleigh number ranging from 6.84 × 106 to 9.79 × 108. Its cooling is done through porous media saturated by Water-ZnO nanofluid contained in a spherical closed cavity maintained isothermal. Steady state measurements were carried out on an industrial prototype at scale 1:1 using various porous media whose thermal conductivity, relative to that of the base heat transfer fluid (water), varies between 0 (without porous medium) and about 40. These porous media are saturated with a water-based ZnO nanoparticles nanofluid with a volume fraction ranging from 0 (pure water) to 10%. The average Nusselt number determined for different combinations of the three influencing parameters confirms that saturated porous media enhances natural convective heat transfer. Influences of Rayleigh number, volume fraction and thermal conductivity ratio have been quantified. Results are in agreement with those of recent numerical approaches done by means of the control volume method in the 6.51 × 106 - 1.32 × 109 Rayleigh number range. Measurement-calculation deviations are of about 5% on average. They confirm the validity of the model implemented in the numerical approach. The new results of the present work can be applied to various engineering fields such as electronics to optimize thermal design of electronic assemblies and improve their reliability.

Heat transfer and structure of pulsating flow behind a rib

The paper elaborates on experimental studies of heat transfer and kinematic structure of steady and pulsating air flows past a spanwise rib. The forcing frequency range of (0–30) Hz is considered, and the normalized forcing amplitude of velocity is 0.5. Forced flow pulsations are shown to enhance heat transfer if compared to the steady-state flow, particularly in the near-wake region behind the rib. Experimental data on the heat transfer coefficient in pulsating flows are best generalized using the non-dimensional pulsation frequency (Strouhal number). Profiles of velocity and turbulent parameters are demonstrated at a number of representative coordinates along the separation region. The dynamics of the kinematic structure of pulsating flows is described using the profiles of parameters in different phases of forced flow pulsations. The correlation between local heat transfer in the separation region and local hydrodynamic parameters is analyzed. The distribution of heat transfer coefficient over the wall in the rib wake correlates best with the distributions of transverse velocity component.

Experimental study of flame heat transfer in a vertical turbulent wall fire

This paper presents experimental measurements in a vertical turbulent wall fire, with a primary goal of developing a deep understating and database for heat transfer on solid surfaces where fuel pyrolyzes. An enhanced porous-plate vertical wall-fire burner was developed, and experiments were conducted with various mass transfer rates of ethylene to measure flame temperature, radiative power, spectral emission, radiative and convective heat flux. The maximum mean flame temperature is found to decrease with height, and the temperature profile in the outer flame boundary layer to be self-similar. The radiant power and heat flux show a linear increase vertically in the pyrolysis region and the peak shifts to higher evaluation with a greater fuel flow rate. Soot emission contributes a dominant fraction to thermal radiation. On the other hand, the convective heat flux promptly reduces from the flame leading edge in the laminar region at the bottom of the wall-burner and remains relatively steady in the downstream turbulent region. Convective heat flux dominates the heat transfer in the lower height while radiative heat transfer is more prominent in the upper region of the wall.

Mixed Convection Heat Transfer from a Short Vertical Cylinder Placed in a Cross Flow

Experimental study of mixed free and forced convection heat transfer from a vertical heated cylinder placed in a crossflow is studied for Reynolds number from 1.1×103 to 3.3×104 and Prandtl number 0.7. The average Nusselt number is calculated and correlated with the Reynolds number. An empirical correlation for predicting the overall heat transfer from the cylinder is developed for this case.

Heat Transfer Enhancement in a Double Pipe Heat Exchanger Using Different Fin Geometries in Turbulent Flow

Heat transfer enhancement is widely used for improving heat exchanger performance in industrial processes. This is accomplished by increasing the surface area of the heat exchange surface by using fins or inserts of different geometries. The present work is an experimental study of heat transfer when using different fin geometries for the heat exchange surface in a double pipe heat exchanger. The fin geometries included interrupted rectangular fins, circular fins and helical ribs. The heat transfer coefficient and pressure drop were experimentally determined for a range of Reynolds numbers of hot and cold fluids. The results showed that extending the surface using different fin geometries enhanced the heat transfer coefficient, but was dependent on the Reynolds number of both fluids. The maximum heat transfer enhancement was obtained for a rectangular fin and the minimum was for a circular fin. In the case of the finned tubes, the lowest pressure drop was observed for the circular fin and highest for the rectangular fin.

Unsteady heat transfer at impinging of a single spray pulse with various durations

The results of an experimental study of heat transfer at the inlet of a pulsed multi-nozzle spray onto a vertical surface in the regime of evaporative cooling (Tw = 70°C) are presented. The main attention is paid to the measurement of unsteady heat transfer for a single spray pulse of various durations (ton = 2–50 ms). The experiments were carried out using pure water and its 50% and 96% mixtures with ethanol as the liquid phase. Initially, with short pulses, the heat transfer coefficient increases with increasing its duration and at ton≈ 10 ms it reaches a plateau with a practically constant value despite the fact that the mass of the liquid injected per pulse increases with ton.The same tendency is observed for time-averaged heat transfer and for all studied coolant compositions. The probable cause of this phenomenon is the increase in the liquid film thickness on the heat exchanger surface, which in turn suppresses evaporation processes. The thermal efficiency coefficient η of the pulsed spray is maximal for the shortest pulses, and it decreases significantly with increasing pulse duration.

Experimental study of radiative heat transfer from laminar non-premixed methane flames diluted with CO2 and N2

This paper presents an experimental study about the effect of fuel dilution with inert gases on the radiative heat transfer characteristics of laminar non-premixed coflow methane flames. The visible flame lengths were determined and flickering is characterized when necessary. Radiative heat flux distributions were measured and used to determine the radiative fraction of each flame. In total, 55 flames were studied with powers up to 0.554 kW and with CO2 and N2 dilutions up to 60%. The flames were grouped in series of constant power and constant flow rate. It is shown that the addition of CO2 may decrease or increase the radiative fraction depending on the dilution level and on the characteristic residence time of the flame. Dilution with N2 always leads to a decrease of the radiative fraction. The results are relevant for practical applications as in biogas combustion and also proved to be useful for validation of radiative heat transfer models.

Experimental Study of Heat Transfer in a Direct Injection Diesel Engine

An experimental study has been established to understand the effective cylinder wall heat transfer rate and temperature of a direct injection diesel engine. Temperatures were calculated under a wide range of load at different locations in the cylinder block and cylinder head of the engine using pre-arranged thermocouples to acquire the temperature gradient and consequently realize the equivalent heat transfer rate, cylinder wall temperatures, heat transfer co-efficient and engine speed. Diesel and biodiesel blends (B20 and B100) are used as fuels and the temperature readings are found using a ‘k-type’ thermocouple and temperature readings are noted. Raise in the cylinder temperature is observed as the engine torque increases for the diesel and biodiesel. As the engine speed increases, the exhaust gas velocity involved in and out of the engine will increases and this lead to an increase in the heat transfer co-efficient for diesel and biodiesel.

Numerical and experimental study of heat transfer in a cubic cavity with a PCM in a vertical heated wall

In this work, a transient numerical and experimental study is presented, focused on describing the heat transfer in a cavity filled with air, which has one vertical wall with a phase change material (PCM). The wall with PCM was exposed to constant heat flux, whereas the opposite vertical wall was kept at a constant temperature. For the theoretical study, a model for heat transfer was proposed and solved using computational fluid dynamics software. The enthalpy method was used to simulate the phase change in the PCM. For the experimental study, five temperature profiles were obtained for air, one for PCM, and at different points of shared wall between PCM and air. A comparison was made, between, experimental and numerical data, to determine the predictive capacity of the model. Besides, numerical results of the liquid fraction in the PCM were compared with data reported in the specialized literature. It was found that the enthalpy method was adequate to study proposed in this study. The numerical temperature fields and flow patterns in the air were analyzed, as well as the evolution of the liquid fraction in the PCM. With the validated model, a parametric study was conducted to quantify the effect of Rayleigh number and PCM thickness on Nusselt number and the average air temperature. It was found that the use of the PCM decreased the heat transfer to the air. The comparison with the case without PCM indicated that Nusselt number values reduced between 66.80 and 75.47%.

ЭКСПЕРИМЕНТАЛЬНЫЕ И РАСЧЕТНЫЕ ИССЛЕДОВАНИЯ ТЕПЛООБМЕНА ПРИ КИПЕНИИ ЖИДКИХ МЕТАЛЛОВ НА МОДЕЛЯХ ТВС БЫСТРЫХ РЕАКТОРАХ В АВАРИЙНЫХ РЕЖИМАХ

Studies of the boiling of alkali metals show that, compared with boiling water, the process of boiling liquid metals has significant features. Cooling of a fuel assembly in accident conditions when accident protection is triggered and circulation pumps are switched off (ULOF) leads to the study of fuel elements cooling at reduced coolant flow rates or even tipping circulation in fuel assemblies. There is only limited data on the boiling of sodium in fuel assemblies in these regimes. The results of a series of experiments on heat transfer and stability of circulation during boiling of a sodiumpotassium alloy on single fuel assembly models and in a parallel fuel assembly system with natural coolant circulation performed at JSC “SSC RF - IPPE” are presented. The results of comparing the data of the calculated and experimental studies are presented. The influence of the surface roughness of the fuel rods on the heat transfer and flow regimes during boiling of a liquid metal in bundles is demonstrated. The results of experimental studies of heat transfer during boiling of sodium in natural and forced convection regimes in a fuel assembly model with a “sodium cavity” located above the reactor core intended to compensate for the positive sodium void reactivity effect in emergency situations with sodium boiling are also presented. It is shown that it is possible to provide continuous sodium cooling of fuel element simulators in fuel assemblies under these conditions. The results of the generalization of data on heat transfer during boiling of liquid metals in bundles and a cartogram of the regimes of two-phase flow during boiling of liquid metals in bundles are presented. The objectives of further research are discussed.

Study of Heat Transfer through Radially Buffled Rotating Annulus

This paper presents an experimental study of laminar forced convection heat transfer for cold water flows through annular tube. The test section consisted of two annular spaces the inner of which for the tested cold water while the outer for the heating water flow that heated in a tank by electrical heaters. The inner tube of the annulus is rotated with rotational speeds which varied from 175 to 650 p.m. The outer tube of the annulus have a constant diameter of 54.5mm but the diameter of inner tube of the annulus is equal to 21, 26.7 and 33.5 mm so as to give diameter ratios of 0.62, 0.49 and 0.386. Variable numbers of semi-circle of buffles are used circumferentially around the rotating tube. The diameter of buffle is equal to 52 mm and 2 mm thickness. Cold water flowed axially through the annular space with velocities that ranged from (0.0192 to 0.0666 m/s), to give axial Reynolds numbers in the range (612 ≤ Re ≤ 2175), to cover the laminar flow regime. In the present work the effects of radius ratio, axial Reynolds number and rotational Reynolds number on the heat transfer are investigated. The results show that, the rate of heat transfer of the rotational annular tubes is higher than that of the stationary one of the same radius ratio and axial Reynolds numbers, and the rate of heat transfer of rotational annular tubes with baffles higher than rotational annular tubes without baffles of the same radius ratio and axial Reynolds numbers. 1- Rotation without buffle, Produces a maximum increase in Nusselt number of the order of 79% at Rea= 2080, Reω= 21545 and rotational speed= 480 r.p.m, (increase of only 11% in Nusselt number at Rea=646.2, Reω=7808 ,( rotational speed of 175 r.p.m), compared to the results of the stationary annular tube of the same radius ratio . 2- Rotation with buffle, Produces a maximum increase in Nusselt number of 200% at Rea= 2175, Reω= 21884 and rotational speed= 480 r.p.m, and increase of only 84% in Nusselt number at Rea=634, Reω=21778, ( rotational speed of 175 r.p.m) compared to the results of the stationary annular tube of the same radius ratio . Two correlations are made to describe the relation between Nusselt numbers and axial Reynolds number, diameter ratio, pitch to diameter ratio P/D and rotational Reynolds number and Reynolds number for stationary, rotating and buffled rotating annuli.

Experimental study of nanofluids flow and heat transfer over a backward-facing step channel

The forced convective heat transfer and friction factor of nanofluids flow over a backward-facing step (BFS) in a channel with the base wall dissipating a uniform heat flux are experimentally investigated in this paper. Nanoparticles such as CuO and MgO dispersed in pure ethylene glycol (EG) with average diameters of 40 nm, and volume concentrations of 0, 1, 3, and 5% are considered as the working nanofluids. The results indicate that the heat transfer rate increases as the volume concentration of nanoparticles increased. The Nusselt number is enhanced up to 11% at 0.05 volume concentration compared to pure EG. The friction factor increases up to approximately 15% at Reynolds number (Re) of 5000 and volume concentrations of 0.01 and 0.03. The maximum value of the PEC in a BFS channel is 1.5 for the CuO-EG and 1.2 for the MgO-EG at a volume concentrations of 0.03 and Re = 20,000.

Experimental heat transfer studies on copper nanofluids in a plate heat exchanger

The objective of the present work is to study the influence of copper nanoparticle concentration on heat transfer performance of a mixed base fluid. In the present study, the performance of copper nanoparticles in ethylene glycol (eg) + propylene glycol (pg) + water (W) base fluid was analyzed in the chevron- type plate heat exchanger. The sol-gel method was used to prepare copper nanoparticles (100 nm), dispersed in two different mixed base fluids of volume fractions 5%EG + 5%PG + 90%W and 15%EG + 5%PG + 80%W. Experiments were performed by varying the nanoparticle concentration from 0.2 to 1.0 vol.%. Three different hot fluid inlet temperatures were used (55, 65 and 75 ?C). It is revealed from the study that the rate of heat transfer increased significantly with the mixed base fluid. Result shows that at 75 ?C, 9 and 14.9% enhancement in the Nusselt number is obtained for 5%EG + 5%PG + 90%W and 15%EG + 5%PG + 80%W base fluid, respectively, for the nanoparticle concentration of 1%.

Experimental study of moisture role and heat transfer in thermal insulation fabric against hot surface contact

Understanding of moisture role and heat transfer in thermal insulation fabric was crucial for improving thermal protective performance of the fabric against hot surface contact. The hot contact tester was used to measure the heat transfer in “hot surface-fabric-skin simulant sensor”. Two new indexes were proposed for investigating the effect of moisture content on the transmitted and stored energy. The moisture in the fabric presented different roles for four stages that were divided into sharply increase, stable state, slowly increase and decrease according to the changes of skin temperature. The energy storing efficiencies for these fabrics ranged from 4.1% to 20.7%. The moisture in the fabric increased the energy storing efficiencies, which decreased the heat transfer rate during the heat exposure. The discharging thermal energy previously presented an increase and then decreased over the moisture content. However, the moisture reduced the energy discharging efficiencies, thus enhancing the cooling after the heat exposure. The conclusions from this study would contribute to understanding the moisture effect on the heat transfer for improving the protective level of thermal insulation fabric.

An Experimental Study of R134a Condensation Heat Transfer in Horizontal Smooth and Enhanced Tubes

Abstract In this paper, the condensation heat transfer characteristics of R134a inside enhanced tubes using two types of surface structures with different materials were investigated, which were then compared with plain tubes under the same test conditions. The enhanced tubes were: 1EHTa tube with dimpled and petal arrays structure and 1EHTb tube with protrusion and similar petal arrays structure. The experiment was conducted for a mass flux ranging from 100 to 200 kg m−2 s−1 with saturation temperature of 318.15 K. The inlet and outlet vapor qualities were fixed at 0.8 and 0.2, respectively. The test tubes had the same outer diameter of 12.7 mm. Results showed that the dimpled and protruded surface tubes enhanced the convection condensation heat transfer and the heat transfer coefficient was 1.4–1.6 times higher than that of the smooth tube. Heat transfer enhancement of the 1EHTa and 1EHTb tube was mainly due to the complex roughness surface structures that created swirling and increased the interface turbulence. The condensation heat transfer coefficient increased slightly with increasing mass flux. The pressure drop penalty was found to increase as mass flux increased. Compared with the smooth tube, the pressure drop of Cu-1EHTa tube, SS-1EHTa tube, and Cu-1EHTb tube were 1.15, 1.21, and 1.14 of smooth tube, respectively. Enhanced tubes exhibited higher performance factors (PFs) compared to the smooth tube. The average PF was 1.3–1.5. A new correlation of heat transfer coefficient has been developed within ±15% error band.

Experimental study and ANN-based prediction of melting heat transfer in a uniform heat flux PCM enclosure

Abstract In this paper, heat transfer during the melting process of n-octadecane as a phase change material (PCM) is experimentally studied. This study is followed by an artificial neural network (ANN) to predict the melting characteristics of PCM. Experiments are performed in a rectangular enclosure subjected to a uniform heat flux in one vertical side. Melting heat transfer is characterized by observing the solid-liquid interface and recording the temperature distribution in the enclosure. Experimental results indicate that heat transfer during the melting process is dominated by natural convection. A multilayered perceptron feed-forward neural network trained by the Levenberg-Marquardt algorithm is used to predict the Nusselt number and the melted volume fraction. Rayleigh, Fourier and Stefan numbers are set as input parameters of the network. The optimal structure of the ANN to predict the Nusselt number show high accuracy in estimating the heat transfer characteristics during melting by achieving the mean square error and the correlation coefficient of 4.42 × 10−6 and 0.999, respectively. Based on the proposed ANN, the majority of the data falls within ±6.23% and ±6.54% of the Nusselt number and the melted volume fraction, respectively.