Experimental Study Of Heat Transfer Research Articles

Experimental and numerical study of forced convictive heat transfer in a horizontal circular pipe with varying angles ribs

Experimental and numerical study of forced convictive heat transfer in a horizontal circular pipe with varying angles ribs

Experimental and numerical study of heat transfer and friction factor characteristics of an inclined elliptical dimple channel having inline and staggered pattern

Experimental and numerical studies were carried out to examine the effect of staggered and inline inclined elliptical dimpled plate on Nusselt number (Nu) and friction factor (f) for higher Reynolds number (Re) values. Dimple dimensions of different elliptical dimple pattern were kept constant to compare their thermal performance effectively with the dimple depth (de ) and dimple diameter (dp ) kept as 3 mm and 10 mm respectively for the present study. The features, namely Nu, f and flow structure of the inclined elliptical dimpled plate were calculated for ‘Re’ values ranging from 11,300 to 62,000. The experiment showed that inclined elliptical dimple with staggered and inline arrangement has different effect on flow pattern and heat transfer characteristics. The staggered pattern has 24.8% greater heat transfer and 10.71% lower pressure drop corresponding to inline pattern. The three-dimensional numerical computations clearly show that different test plates have different vortex flow structures and these vortex flow patterns of dimpled plate result in improved heat transfer in comparison to flat plate.

THE INFLUENCE OF DIFFERENT FACTORS ON THE MAISOTSENKO CYCLE EFFECTIVENESS

An important problem in the heat industry is the significant cost of energy resources for air conditioning. Therefore, energy-efficient refrigeration systems based on renewable energy sources and evaporative air cooling, as well as air-conditioning systems according to the Maisotsenko cycle using psychrometric energy of the environment are of great importance. This paper presents the results of experimental and numerical research of the influence of various factors on the efficiency of indirect evaporative heat and mass transfer apparatus on the M-cycle on the basis of a new unit cell.
 An experimental stand of a heat and mass transfer apparatus with vertically arranged plate channels was developed for experimental research. Experimental studies of heat and mass transfer in the new unit cell were conducted in a wide range of Reynolds numbers 200…1200, temperature 16… 30 °С, relative humidity 30… 50 %. The calculated study was performed using a modified ε-NTU method. Studies have shown that the thermodynamic efficiency of a wet bulb thermometer at a Reynolds number below 500 exceeded one. According to the results of computational studies, the influence of cell length and surface intensification of dry channels on thermodynamic efficiency was determined. Shown the effect of changes in thermodynamic efficiency in the range of 86…94 % increase in efficiency by 1 % leads to an increase in cell length by 7 %. The value of the enhancement factor increases with increasing air flow regime, so when the heat and mass transfer surface area increases by 50 % and the Reynolds number 200, the thermodynamic efficiency increases by 14 %, and with the Reynolds number 800 – by 28 %. The dependence of the thermodynamic efficiency of the cells at the dew point on the relative humidity of the inlet air have a maximum, which with increasing Reynolds number shifts toward larger values of relative humidity.
 Unlike traditional air conditioning devices, the M-cycle heat and mass exchanger does not use a steam compression cycle, so energy costs are spent only on the operation of the fan to pump air, which is a more environmentally friendly and energy efficient way of air conditioning.

Effect of Surface Topology on the Apparent Thermal Diffusivity of Thin Samples at LFA Measurements.

This paper deals with the problem of the influence of surface topography on the results of thermal diffusivity measurements when determined using the instantaneous surface heat source method, also called the pulse method. The analysis was based on numerical tests carried out using Comsol Multiphysics software. The results of experimental investigations on the actual material structure using an electron microscope, an optical microscope and a profilometer were used to develop a numerical model. The influence of the non-uniformity of the surface of the tested sample on the determined values of half-time of the thermal response of the sample’s rough surface to the impulse forcing on the opposing flat surface was determined by developing the data for simulated measurements. The effect of the position of the response data reading area on the obtained simulation results was also analyzed. The obtained results can be used to improve the accuracy of experimental heat transfer studies performed on thin-film engineering structures depending on the uniformity and parallelism of the material applied to engineering structures. The difference in half-life determination error results for various analyzed models can be as high as 16.7%, depending on the surface from which the responses of the heating impulse are read. With an equivalent model in which 10% of the material volume corresponds to the rough part as a single inclusion, hemisphere, the error in determining thermal diffusivity was equal to 3.8%. An increase in the number of inclusions with smaller weight reduces an error in the determination of thermal diffusivity, as presented in the paper.

Molecular dynamics simulation and experimental study of heat transfer and phase change of water with slit effect

The condensation water-bridge phenomenon of the heat exchanger surface fins is a common undesirable phenomenon in refrigeration systems and heat pump systems, which would significantly affect the heat exchanger's thermal efficiency due to insulation and obstruction of airflow. Based on the lack of experimental studies on louvered-fin heat exchangers and surface energy for fin-surface condensation, this paper experimentally investigates the effect of wettability (surface energy) on the condensation mechanism and heat exchanger performance. Firstly, hydrophilic and hydrophobic coatings are formed by reagent deposition, which regulates the surface energy of the fins. Experiments are designed to investigate the effect of surface energy on heat exchanger performance at different temperatures, humidity, and wind speed. Finally, the key issues such as condensation morphology are discussed in detail by molecular dynamics simulation, thus filling the gap and providing a reference for heat exchanger surface construction. The experimental results show that the wet condition has a faster condensation rate, with an average rate reduction of 33.2%. Both hydrophilic and hydrophobic fins could improve the performance of the heat exchanger compared to bare fins due to their inherent uniqueness. The hydrophilic fins have a smaller pressure drop, maintain a more stable heat transfer rate, and improve the heat transfer performance by 13.4% in a typical environment (60%) or with lower water content. While the hydrophobic fins show advantages in a wet environment (80%), improving the heat transfer performance by 17.8% due to the severe deterioration of the bare heat exchanger performance. Further, molecular-scale condensation patterns were analyzed, and condensation rates and adsorption forces were quantified. The experimental and simulation results guide the construction and selection of surface energy to optimize the heat transfer surface.

Numerical and experimental study of the transient conjugate heat transfer and fluid flow of passive residual heat removal heat exchanger

The transient heat transfer performance of passive residual heat removal heat exchanger (PRHR HX) with C-tube bundle directly influences nuclear reactor safety under accident condition. This paper presents an investigation of transient conjugate heat transfer and fluid flow phenomena and characteristics of scaled-down single C-tube submerged in water tank under atmospheric pressure at the initial period before saturation pool boiling occurs. Empirical calculation model, experimental setup and numerical model were established separately, and both numerical and experimental studies were carried out. The distributions of tube outlet temperature and water temperatures at horizontal and vertical monitoring points in tank were obtained. Detailed comparisons of the results show good agreements. Also, the change tendencies of heat flux, heat transfer coefficients and heat load were analyzed. The fluctuation of heat flux along normalized length from tube inlet firstly increases but finally tends to keep stable with the change of temperature difference and transformation of heat transfer mechanism. The heat transfer coefficient inside tube shows a slight downtrend along normalized length from tube inlet, but the heat transfer coefficient outside tube shows obvious change in the transient process. The total heat load continuously decreases with time. Furthermore, the volume fraction of vapor is relatively low during the subcooled boiling period. The two-phase flow enhances the agitation in water tank and the fluid velocity near tube outside wall is much larger.

Experimental, comparative and statistical study of heat transfer and drag reduction of Water/Polyisobutylene/nanoSiO2 poly-nanofluid through a horizontal pipe

Turbulent flow of fluid samples (polymeric solution, nanofluid and Poly-nanofluid (PNF)), into a horizontal pipeline was investigated to achieve heat transfer properties and drag reduction. Water, Polyisobutylene (PIB) and nanoSiO2 were the materials used in this study. PNFs prepared by dispersing SiO2 nanoparticles (with concentration range of 0.1–1 wt.%) into dilute Water/PIB solution (with concentration range of 10–50 ppm). The effect of polymer and nanoSiO2 presence on heat transfer properties such as convective heat transfer coefficient and Nusselt number were evaluated and Reynolds number changes were studied in this research. At the same time the effect of concentration and Reynolds number on the Nusselt number and drag reduction evaluated by utilizing a statistical method called analysis of variance (ANOVA).

Experimental study of heat transfer in tube-type radiator of hybrid cooling tower

A hybrid cooling tower is a combined evaporative and dry cooling tower. The authors developed the hybrid cooling tower with inclined corrugated contact elements and the tube-type radiator for cooling the circulating water of industrial plants. The work aims to study heat transfer in the radiator of the cooling tower experimentally. It was found that an increase in the mass flow rate of cooled water in the radiator in the range of (6.43–37.84)10−3 kg/s leads to an increase in the ratio of the amount of heat transferred through the radiator wall to the amount of heat transferred in the evaporation part of the cooling tower by an average of 9.82 times. On the other hand, an increase in the average velocity of cooling air in the range of 1.73–4.09 m/s leads to a decrease in the proportion of heat transferred through the tube wall to the amount of heat transferred in the evaporation part of the cooling tower by an average of 1.55 times.

Experimental Study of Flow Boiling Heat Transfer During Downflow of Refrigerant R-21 in Assembly of Perforated-Wall Minichannels

Experimental Study of Flow Boiling Heat Transfer During Downflow of Refrigerant R-21 in Assembly of Perforated-Wall Minichannels

Conjugate turbulent natural convection with thermal radiation in a vertical and inclined wall with heat sources

This work presents an experimental and numerical study of turbulent conjugate natural convection heat transfer in a plate for vertical and inclined positions containing protruding heat sources mounted on a conductive substrate, with thermal radiation effects. Experiments were performed with seven aluminum protruding heat sources uniformly mounted over an upper substrate fiberglass plate. Insulation and a guard heater were used at the back of the substrate plate. The temperature was measured for each power input and downward inclination angle. Numerical simulations were performed to explore the effects of conductivity, upward and downward inclinations, and heater emissivity. The conjugate problem was numerically addressed for a two-dimensional model, steady-state and incompressible flow under turbulent flow conditions. The main effort was to investigate the effects of power dissipation, conductivity plate-fluid ratio, plate inclination angle, and emissivity of the heater on temperature and Nusselt number distribution. The range of the modified Rayleigh number covers the values between 9 × 105 and 1.3 × 1010. The results found that heat loss by radiation was greater than 35%, the heat transfer became less efficient as the downward angle increased, and a jump in the average Nusselt number was found for upward cases due to plume detachment. A correlation for the Nusselt number was also developed.

Experimental study of two-phase heat transfer of droplet impact on liquid film

Spray cooling is an effective heat transfer technique capable of handling high heat fluxes and temperatures. The study of droplet impact offers a simplified approach to understanding the fundamentals of spray cooling and elucidates the effects of droplet diameter and velocity. Experiments on a single-droplet impact on a thin liquid film with boiling bubbles are carried out in this study. Two-phase interface evolution is observed using high-speed video, and the heater surface temperature variation at the impact point following an impact is recorded and analyzed, which can be divided into the response stage, the recovery stage, the steady stage, and the slow recovery stage. Furthermore, a cooling factor based on the change in heater surface temperature is introduced to evaluate the cooling effect of droplet impact. Results show that the steady stage plays a significant role in the heater surface cooling process. Moreover, the maximum cooling factor increases appreciably with increasing droplet diameter and impact velocity.

An Experimental Study of In-Cylinder Heat Transfer from a Pressurized Motored Engine with Varying Peak Bulk Gas Temperatures

<div class="section abstract"><div class="htmlview paragraph">The variation of in-cylinder heat transfer with parameters such as engine speed, air-to-fuel ratio, coolant temperature and compression ratio were frequently studied in classical research. These experimentally-obtained relationships are important for improving in-cylinder heat transfer models, essential in developing CO<sub>2</sub> reducing strategies. In this publication, a 2.0 liter compression ignition engine was tested in the pressurized motored configuration. This developed experimental setup allowed testing of the engine at speeds ranging between 1400 rpm and 3000 rpm, with peak in-cylinder gas pressures from 40 bar to 100 bar. The engine was motored using different gas compositions chosen specifically to have ratios of specific heats of 1.40, 1.50, 1.60 and 1.67 at room temperature. This enabled motored testing with peak in-cylinder bulk gas temperatures ranging from 700 K to 1500 K. This wide variation of peak bulk gas temperature was achievable even at constant peak in-cylinder gas pressure, which gave the possibility of varying the thermal load of the engine independently from the gas pressure load. This experimental setup offered the repeatability and robustness of motored testing, with the benefit of a fired-representative gas pressure and thermal load. Throughout the test matrix, the engine was instrumented with eroding surface thermocouples at two locations in the cylinder head; at the cylinder central axis, and at the periphery in the squish region. The steady-state and transient components of heat flux were investigated separately, along with the average surface temperature and its swing. The transient component of heat flux was computed using the Impulse Response method, coupled with a two-dimensional finite element model of the eroding thermocouples, as presented in SAE 2021-24-0018. This method takes into account the two-dimensional nature of heat flux through the thermocouples and hence presents a more robust analysis than the more common one-dimensional treatment using the Fast-Fourier Transform method.</div></div>

Experimental study of turbulent flow heat transfer and pressure loss over surfaces with dense micro-depth dimples under viscous sublayer

Presented are experimentally-measured heat transfer and pressure loss characteristics for dimpled surfaces, placed along one surface of a channel, with different ratios of dimple depth to channel height δ/H, and for Reynolds numbers ranging from 10,000 to 70,000. With the same relative dimple spacing to dimple print diameter, and the same ratio of dimple depth to dimple print diameter of δ/d = 0.20, experimental results for arrays of spherical indentation dimples with micro depths of δ = 0.6 and 1.0 mm (δ/H = 0.03 and 0.05) are compared with thermal characteristics for arrays of larger dimples with δ = 4.0 mm and δ/H = 0.20. Local surface heat transfer distributions are measured using transient liquid crystal thermography. Additional steady-state experiments are also employed to provide overall heat transfer enhancement and friction factor augmentation values. Results indicate that Nusselt number ratios and friction factor ratios of a turbulent flow in the channel are dependent upon the ratio of dimple micro depth to channel height, which are different findings from previous studies with larger-depth dimples in the cooling channels. As such, the present data for micro-depth dimples with ratios of dimple depth to channel height δ/H of 0.03 and 0.05 are firstly provided. The importance of a critical Reynolds number is demonstrated by the results, which is determined when the ratio of viscous sublayer layer thickness to the dimple depth is equal to unity, and has been ignored in the previous dimpled channel heat transfer design. Nusselt number enhancements for the micro dimples are generally lower at lower Reynolds numbers, but increase as the Reynolds number becomes larger and approaches the critical Reynolds number. Nusselt number enhancements are then approximately invariant with Reynolds number, when Reynolds number then exceeds the critical value.

Experimental study of heat transfer during the flow of a gas coolant in a heated quasi-triangular channel

The paper considers experimentally the process of heat transfer during the flow of gas mixtures in a heated thin-walled quasi-triangular tube. The cross-section of the tube corresponds to the shape of the channel formed by the dense packing of the reactor rods for generating heat at a power plant. The working fluids were a helium-xenon gas mixture with a Prandtl number of 0.23 as a promising coolant for a nuclear installation generating electricity, and air with a Prandtl number of 0.71. Temperature distributions of the heated wall are obtained in a wide range of flow velocities, and the effect of laminar-turbulent transition at the initial section of the tube and high-velocity turbulent flow at the outlet on these distributions is analyzed. An approach for calculating local heat transfer coefficients for an unsteady flow, based on determining the local velocities of the coolant in the channel, is proposed. Significant non-uniformity of the heat transfer coefficient along the channel length was obtained with a decrease in its value at the inlet area and an increase towards the outlet due to flow acceleration. An increase in the heat transfer coefficient during the flow of helium-xenon mixture is more intense than in the air flow. It is shown that this circumstance should be taken into account when designing compact power plants.

Experimental and numerical study of heat transfer and pressure loss in a multi-convergent swirl tube with tangential jets

This paper presents a comparative experimental and numerical study of heat transfer and pressure loss in a swirl cooling system with multi-convergent tubes. The results are compared with those of a baseline circular swirl tube. The two swirl tubes are both with multiple tangential jet slots, which can induce large-scale swirling flows and significantly enhance the convective heat transfer. The multi-convergent swirl tube can be considered as an effective internal cooling method for the gas turbine blade leading edge. Transient liquid crystal thermography is used to obtain detailed heat transfer distributions on the internal surfaces of the two swirl tubes. It is shown that through a small change in the swirl tube structure the heat transfer rates and uniformity could be significantly improved in the multi-convergent swirl tube. In more detail, the heat transfer of the multi-convergent swirl tube reaches peak values under the jet slots, and then decays more slowly than the baseline swirl tube. After the third jet, the heat transfer of the convergent sections between the inlet jets keep almost stable along the axial direction, since the convergent structures restrain the decay of swirl intensity. Besides, the multi-convergent structure offers a swirl tube with the ability to better resist against the cross flow, which guides the upstream swirling flow to the downstream tube core, avoiding the collision and mixing between the cross flow and the downstream wall jet flow, thus improving the heat transfer rates. Compared with the baseline swirl tube, the experimental globally averaged Nusselt numbers of the multi-convergent swirl tube can be increased by 11.8%-23.3% for the Reynolds number range of 10,000 to 40,000. As a supplement to the experiments, three-dimensional numerical computations provide more insights into the turbulent flow structure in the two kinds of swirl tubes.

Turbulent heat transfer in a liquid metal tube flow with azimuthally inhomogeneous heat flux

An experimental study of the convective heat transfer in a turbulent liquid metal tube flow with azimuthally inhomogeneous heat flux is presented. Prior to the liquid metal experiments, the validation of the test section was realized using water. These results showed a very good agreement with literature data. For the liquid metal experiments, an eutectic alloy of gallium, indium and tin (GaInSn) was used. The Péclet number was varied between 1400 and 3600, thus in a regime of forced convection. Experiments with homogeneous heating over the full circumference of the tube and inhomogeneous heating over half of the circumference with the other half being insulated, were performed. The azimuthally averaged Nusselt number and the temperature distribution in the tube wall were investigated. The results suggest that the azimuthally averaged Nusselt number for water and liquid metal tube flows with inhomogeneous heating over the circumference can be calculated sufficiently well with literature correlations for uniform heat flux. For an inhomogeneous heat flux the azimuthal temperature gradient in the tube wall increases for higher Reynolds number and is more pronounced for GaInSn than for water. Furthermore, impurities like oxide particles significantly decrease the liquid metal convective heat transfer coefficient.

Experimental and Numerical Study of Heat Transfer and Pressure Loss in a Swirl Multi-Pass Channel With Convergent Jet Slots

Abstract This paper presents a comparative experimental and numerical study of the heat transfer and pressure loss in a swirl multi-pass channel with tangential jet slots, and another baseline multi-pass channel with 180-deg U-bends as a comparison baseline has also been investigated. The tangential jets can induce large-scale swirling flows and significantly enhance the connective heat transfer. The swirl multi-pass channel can be considered as an effective internal cooling strategy for the gas turbine blades. Transient liquid crystal thermography is used to obtain the detailed heat transfer distribution on the internal surfaces of the multi-pass serpentine channels. The heat transfer patterns in the swirl multi-pass channel are quite different from those of the baseline multi-pass channel. Circumferentially, the three heat transfer surfaces, i.e., the bottom, top and side surfaces of the two multi-pass serpentine channels, have roughly similar trend along the flow direction and reach the peak values after the U-bends and jet injections, respectively. Compared with the baseline multi-pass channel, the experimental globally averaged Nusselt number ratios of the last two passes in the swirl multi-pass channel can be increased by up to 82.9%, 104.8%, and 124.6% for the Reynolds numbers 20,000, 40,000, and 60,000, respectively. The differences of globally averaged Nusselt number ratios between the two kinds of multi-pass serpentine channels gradually increase with the Reynolds numbers, which means that the swirl multi-pass channel has higher heat transfer enhancement capability at a higher Reynolds number. The circumferentially uniform heat transfer characteristic of swirling flow also performs well in this study. The high and circumferentially uniform heat transfer is mainly due to the large-scale swirling flow induced by the tangential slots. In more detail, the large-scale swirling flow impinges onto the surface and further induces high tangential velocity near the wall, which destroys the boundary layer flow and thus improves the heat transfer rates at the wall. However, the notable pressure loss of the swirl multi-pass channel should be further controlled reasonably, which is about 5.4 times that of the baseline multi-pass channel. As supplements to the experiments, three-dimensional numerical computations provide more insights into the turbulent flow structure in the two kinds of multi-pass serpentine channels.

Experimental study of heat transfer between fluid flowing through fracture surface with tortuous seepage path

Under the dual challenges of energy demand and environmental protection, geothermal energy has broad application prospects because of their cleanliness and huge reserves. Understanding the flow and heat transfer process of fluid flowing through fractured rock is of importance for the utilization of geothermal resources. A more practical calculation equation for convective heat transfer coefficient is derived based on Newton's cooling equation and previous research. Samples with different seepage paths were prepared through 3D printing technology and cement mortar pouring method. The effect of seepage path tortuosity, aperture, and initial temperature on the characteristics of heat transfer were analyzed and discussed by conducting the seepage and heat transfer test. The experimental results show that the overall convective heat transfer coefficient of sample with seepage path are lower than those of sample with smooth fractures. The greater the tortuosity of seepage path, the greater the overall convective heat transfer coefficient. In addition, the correlation of tortuosity and C, n in the characteristic number equation is proposed, providing significant parameters for the study of hydraulic-mechanical-thermal coupling when fluid flowing through rock mass with high temperature.

Experimental study of condensation heat transfer of R134a inside the micro-fin tubes at high mass flux

The flow condensation heat transfer coefficient of R134a inside two helical micro-fin tubes with outer diameter of 6.35 mm and helical angles of 18o and 28o respectively were experimentally studied at a high mass flux ranging from 500 to 1100 kg m−2 s−1. The condensation temperatures are 35°C, 40°C and 45°C, and the vapor qualities ranges from 0.95 to 1.00 at the inlet section and from 0.00 to 0.05 at the outlet section. Experimental results show that there are annular and intermittent flow regimes inside the tube under experimental conditions, the ratio of annular flow region to the total heat transfer region is about 70%. The liquid film flow changes from rotational flow to coexistence of rotational and horizontal flows when the vapor quality is 0.475±0.08, 0.486±0.06 and 0.521±0.05, respectively and the corresponding saturation temperature is 35°C, 40°C and 45°C. Based on the experimental data, the heat transfer coefficient increases with increasing mass flux and fin helical angle, and with decreasing saturation temperature and Reynolds number of cooling water. In addition, the heat transfer coefficient of annular flow is greater than that of intermittent flow, in other words, the heat transfer coefficient increases with increasing vapor quality. The heat transfer coefficient obtained at high mass flux was compared with the calculated data of some published correlations used at low mass flux. It was found that most correlations underestimate the experimental heat transfer coefficient inside the micro-fin tube, and these prediction deviation increases with increasing mass flux and fin helical angle. By introducing some dimensionless numbers as well as considering the flow mechanism of liquid film inside the tube, two heat transfer coefficient correlations were proposed based on the experimental data. The ratio of liquid film thickness to fin height was used to characterize whether the liquid film flow is rotational flow or coexistence of rotational and horizontal flows in the proposed correlation, which shows a better prediction accuracy with a mean deviation of less than 7.3%.

Experimental study of fire propagation and heat transfer of biomass straw fuel with different stacked diameters and thicknesses

The combustion of biomass straw that ignited by a fire point is an important latent danger form of wildfire. The investigation of development of the straw burning are essential for the efficient and safety utilization of biomass straw as a typical fuel. In this paper, the naturally stacked straws with diameters of 60 cm, 90 cm, and 120 cm and thicknesses from 2.0 cm to 12.2 cm were ignited from a point-source fire. The fire propagation and combustion characteristics including propagation stage, flame spreading rate, mass loss rate (MLR), and flame central temperature were measured and analyzed. The results showed that the straw combustion process was divided into four typical stages, namely the center combustion stage, merged annular stage, separated annular stage, and decay stage. The combustion area between the inside and outside flame front forms an annular shape, thus forming a hollow structure in the flame. The outside flame front spreads outward linearly with the spreading rate approximately rising with the increase of diameter and thickness. The peak of MLR is determined by diameter and thickness, hence an MLR quantitative model was constructed as m′/SL̇″h∝Deq2.391 . The exponential function between the dimensionless flame temperature Tf-T∞T∞ and the z-z0ṁ-2/5 was established. The inclined trend was becoming more obvious with a smaller negative slope in the three longitudinal regions.