Values Of Heat Transfer Coefficient Research Articles

Periodic heat transfer characteristics of additively manufactured lattices

In the present study, the flow and heat transfer characteristics of lattices with four different unit cell topologies, namely, (a) Octet, (b) FD-Cube, (c) Tetrakaidecahedron (TKD), and (d) Cube were investigated with air as the working fluid. All the samples had the designed porosity of ∼ 0.88 and unit cell size of 10 mm. The distribution of local heat transfer coefficient revealed that depending on the unit cell topology, the flow became thermally developed after travelling over five or six rows of unit cells in streamwise direction. The heat transfer coefficient values in thermally developed zones were similar for Octet, FD-Cube and TKD. Overall heat transfer coefficient provided by Octet, FD-Cube, and TKD was in the range of ∼216 W/m2K to 415W/m2K and Cube yielded the lowest heat transfer coefficient values between 167W/m2K and 290 W/m2K.Three different characteristic length scales, viz. (a) channel hydraulic diameter, (b) pore-diameter, and (c) fiber-diameter were used to define the Nusselt number. The usage of pore-scale dimensions such as pore-diameter and fiber-diameter provided a very coherent data set for Nusselt number of all the topologies which helped in deriving a single correlation for predicting the thermal performance. The lattices provided heat transfer augmentation of 10-20 times higher than the smooth channel counterparts. Amongst all the investigated topologies, FD-Cube incurred the highest and Cube yielded the lowest pressure drop. It was also observed that convective heat transfer coefficients obtained for different lattices can be conveniently reported in Nusselt number forms with pore diameter and/or fiber diameters as reference length scales, where such Nusselt numbers for different lattice configurations converges to a single trendline when plotted against Reynolds numbers (also based on same reference length scales). We have provided a single correlation for Nusselt numbers as a function of Reynolds numbers which fits all the lattice configurations studied here. Finally, the thermal-hydraulic performance of lattices is reported which had similar magnitude levels for all the investigated topologies and a single correlation has been proposed for THP covering all the configurations of present study. The investigated topologies provided THP of ∼ 3 at the lowest investigated Reynolds number.

Performance Optimization of the Helical Heat Exchanger With Turbulator

In this paper, optimization on a two-tube helical heat exchanger with a fin is represented. The spiral pipes heat exchanger which is made of the cooper is adopted for investigation. The effects of three types of fins with the proposed geometric shapes on the overall heat transfer coefficient and pressure loss are investigated. The fins are located on the inner surface of the outer pipe. The obtained numerical results are compared with the experimental results, and a good agreement is observed between the results. The studies show that the total heat transfer coefficient has increased by 170% compared to an exchanger with no fin. Therefore, the best fin has been selected based on the benefit-cost-ratio (BCR) factor. Finally, using the new represented optimization algorithm, the height of the represented triangular fin is optimized to represent the best values for overall heat transfer coefficient and pressure loss of the helical heat exchanger. In addition, the results indicate that reducing the density and height of the triangular fin increases heat transfer and reduces pressure loss.

Numerical study on the condensation characteristics of various refrigerants outside a horizontal plain tube at low temperatures

The condensation of refrigerants outside the tubes of liquefied natural gas (LNG) at low temperatures is the main factor affecting the performance of the LNG intermediate fluid vaporizer (IFV). In this paper, a CFD model is established to investigate the condensation heat transfer characteristics of refrigerants outside a horizontal plain tube at low temperatures over a wide range of wall subcooling. The model, based on the combination of the VOF multiphase model and Lee phase change model, agrees well with the experimental data. The transient condensation heat transfer coefficient is obtained and the periodic dripping characteristics of the condensate are analyzed. Due to the decrease of the interfacial mass transfer with the pressure decreasing, there is a peak of the periodic-averaged heat transfer coefficient with the variation of the saturation temperature, which is different from that of the Nusselt's prediction. The saturation pressures corresponding to the peak values of the periodic-averaged heat transfer coefficients at the subcooling of 10 °C are all in the range of 0.35–0.65 MPa for the four different refrigerants. In addition, the surface tension plays a positive role in the heat transfer process as the condensate film is pulled towards the wall and thinned. In view of the heat transfer performance along the whole LNG tube, dimethylether (DME) and butane can be considered as the intermediate fluids in the future design of the IFV by taking account of the comparison with six different refrigerants.

Experimental and numerical analyses of thermal-hydraulic characteristics of aluminium flying-wing fins

• In general, the flying-wing fin performs better than the wavy fin at the same Re . • The field synergy of the flying-wing fin is much better than that of the wavy fin. • There exist the butterfly-shaped low velocity zones perpendicular to the streamwise direction. • The heat transfer coefficient on the obtuse-angled side of the fin is larger. • Both fin length and fin thickness have little effect on the performance of the fin. The secondary heat transfer surface of the shovel-cut finned tube, called the flying-wing fin, eliminates the thermal contact resistance. In the present study, the thermal-hydraulic characteristics of the flying-wing fin at Reynolds number range of 1500–3000 were studied, including overall quantitative analysis and three-dimensional thermophysical field analysis. It was found that the ratio of Nu · η 0 / f 1/3 of the flying-wing fin (Case A1) is about 8.6% larger than that of the wavy fin (Case B3). The fundamental reason is that the flying-wing fin has a smaller average field synergy angle than the wavy fin. There is a butterfly-shaped low-velocity zone at the root of the wave trough of the monitoring section on the acute-angled side of the flow channel. In addition, along the fin height direction, the influence range of this low-velocity zone on the flow field is less than around 1.34 mm. Similarly, there are butterfly-shaped zones for the temperature and field synergy angle distributions. The average value of the convective heat transfer coefficient on the left side of the flying-wing fin is greater than that on the right side. In general, the promising flying-wing fins show better thermal-hydraulic performances than the wavy fins with similar geometric parameters, which deserve further promotion and engineering application.

Influence of architectural features of buildings on heat losses from their facades

In the heat balance of buildings, the main heat loss occurs through enclosing structures. Determining the heat transfer coefficients on the exterior facades of buildings is not an easy procedure, as they depend on a wide range of parameters: wind speed in the surface atmosphere, wind direction relative to the orientation of facades, surface orientation relative to wind, surface angle relative to the ground plane, type terrain, the impact of adjacent buildings, surface texture, surface and air temperature differences, surface size and aspect ratio, etc. The paper presents the results of a numerical study of heat transfer from the outer surfaces of separately located structures in the surface layer of the atmosphere. Schematic models of the building have the same heat transfer surface area, but different heights and lengths. CFD-modeling of heat transfer of buildings in the wind flow in a three-dimensional setting allowed to establish some features of heat transfer of enclosing structures depending on the height of the building and the characteristics of the wind flow. In particular, it is shown that the average surface values of heat flux density increase with increasing building height. The results of the calculation of the average heat flux density on the surface of buildings indicate its dependence on the height of this building. This dependence is observed only for houses with a height of less than 30 m. For taller houses, this dependence is insignificant. The local values of heat transfer coefficients on the windward and leeward facades of the building obtained as a result of modeling indicate incorrect application of heat transfer coefficient values αк = 23 W / (m2 ∙ K) for construction practice for external surfaces of enclosing structures. For windward facades, a polynomial model for the heat transfer coefficient is more suitable, and for leeward ones, the Frank formula for αk.

Effects of tip gap on transonic turbine blade heat transfer characteristics with pressure side film cooling

Investigated are local, spatially-resolved and line-averaged distributions of surface adiabatic film cooling effectiveness and surface heat transfer coefficient ratios for a transonic turbine blade tip. The tip contains a squealer rim, and a single row of film cooling holes is located on the pressure-side of the blade very near to the blade tip. A two-dimensional linear cascade, with four flow passages and five complete blades, is utilized for the investigation. Associated data are provided for different film cooling flow conditions for two different tip gaps, which correspond to 1.16 and 0.66% of true blade span. Experimentally-measured, spatially-resolved surface data demonstrate that improved surface thermal protection on the squealer tip surface, from pressure side film cooling, is often present with a smaller tip gap. Such a conclusion is illustrated by heat transfer coefficient values at many surface locations which are generally lower, and by adiabatic film cooling effectiveness magnitudes which are often substantially larger for the smaller tip gap, relative to values associated with the larger tip gap. Associated effectiveness values, along the upper pressure side, vary in a significant manner as blowing ratio changes. Along the tip region of the blade, for both tip gap values, line-averaged adiabatic film cooling effectiveness generally increases with increasing blowing ratio for the pressure side rim. Effectiveness data for the suction side rim with the smaller tip gap, show more complicated variations as blowing ratio changes. Heat transfer coefficient data along the squealer tip surface and on the upper pressure side of the blade surface generally have variations which are similar to the baseline blade with no film cooling for many surface locations. The local deviations, which are present, are due to locally augmented mixing and shear, and increased turbulent transport from the advective presence of the film coolant.

Use of segmental baffle in shell and tube heat exchanger for nano emulsions

AbstractAmong the heat exchangers (HE), the shell and tube type is being widely used in different applications like oil, chemical, and power plant Industries. The incorporation of segmental baffles (SB) improves the HE capacity from higher temperature fluid to lower temperature fluid. Nanofluids can be effectively used to enhance the heat transfer rate. In this study, numerical simulations have been carried out in a shell and tube heat exchanger (STHX). Among HE design methods, Tubular Exchanger Manufacturers Association (TEMA) standard is being used for better design by many researchers. In this paper, the computational fluid dynamics analysis was carried out with Al2O3, CuO, and SiO2 nanofluids amid 1, 3, and 5 vol. % with water emulsion to enhance the heat transfer coefficient of STHX. The nanofluid has been used in the cold fluid of the HE and on the other side hot water is used. From the results, it is noticed that with the increase of Nanofluids, the value of heat transfer coefficients is found to be increasing. The overall heat transfer coefficient has been enhanced for Al2O3, CuO, and SiO2 about 10.41%, 12.27%, and 9.56%, respectively, at 0.22 kg/s for the 5 vol. % addition. It is also depicted that the pressure drop is increasing with the incorporation of nanofluids.

Heat Transfer Modelling and Simulation of a 120 mm Smoothbore Gun Barrel During Interior Ballistics


 
 
 Understanding the heat transfer phenomenon during interior ballistics and consequently presenting a realistic model is very important to predict the temperature distribution inside the cannon barrel, which influences the gun wear and the cook-off. The objective of this work is to present a new detailed numerical model for the prediction of thermal behaviour of a cannon barrel by combining PRODAS interior ballistics simulation with COMSOL simulation. In this study, a numerical model has been proposed for the heating behaviour of a 120 mm smoothbore cannon barrel, taking into account the combustion equation of the JA-2 propellant. Temperature dependent thermophysical properties of product gases were used for the calculation of the convective heat transfer coefficient inside the barrel. Projectile position, velocity of the projectile, gas temperature inside the barrel, volume behind the projectile and mass fraction during interior ballistics have been obtained by PRODAS software and used in the numerical model performed by COMSOL multiphysics finite element modelling and simulation software. Temperature simulations show that maximum wall temperature inside the cannon barrel is observed after 3 ms from fire, when maximum value of the convective heat transfer coefficient inside the barrel is observed. The results reveal that the convective heat transfer coefficient of burned gases inside the gun has major effect than the burned gas temperature on the heat transfer phenomenon.
 
 

Variation of heat transfer characteristics of a microchannel heat sink with geometry

In this paper, variation of heat transfer characteristics has been studied for a microchannel heat sink. A curvature has been added to the geometry of rectangular microchannel heat sinks to improve its heat transfer characteristics. Simulations have been performed by solving the governing equations along with boundary condition using the Ansys Fluent. Two types of cross sections, characterised by their hydraulic diameters, and a range of reynold’s number varying from 100 to 800 has been used in this study to understand the effects of geometry on heat transfer. The results of the curved and rectangular microchannels have been plotted and compared with each other. From the results, it was observed that by adding a curvature to the microchannel, better heat removal performances can be obtained. Curved channels showed better performance in the temperature gain between the inlet and outlet, and have a higher value of heat transfer coefficient. Further, the channel having less hydraulic diameter has better performance than the one with more hydraulic diameter.

Numerical investigation on flow and heat transfer characteristics of supercritical methane–ethane mixture in a straight channel

Printed Circuit Heat Exchanger (PCHE) is considered as a promising heat exchanger for offshore Liquefied Natural Gas (LNG) production due to its compactness and high efficiency. To reveal the flow and heat transfer performance of real component nature gas mixture in PCHE, numerical study was conducted to obtain flow and heat transfer characteristics of supercritical methane–ethane mixture in a straight channel of PCHE. The influence of operating parameters including inlet temperature, mass flux and outlet pressure are investigated. The simulation results show that heat transfer coefficient and pressure drop increase with the increase of inlet temperature and mass flux, and decrease with the increase of outlet pressure. The overall variation tendency of heat transfer coefficient and pressure drop in supercritical methane–ethane mixture flow was similar to those in supercritical methane flow, but there still exists some difference due to their different physical properties. For the value of heat transfer coefficient, supercritical methane flow is about 7% larger than that of supercritical methane–ethane mixture flow. And for frictional pressure drop, supercritical methane flow is much larger by about 12%. Finally, new correlations were proposed for supercritical methane–ethane mixture flow, which are helpful for a more accurate flow and heat transfer calculation in PCHE designing for natural gas.

Pool boiling of R-134a refrigerant over a thin film aluminum nanostructured coated surface

Experimental investigation of nucleate pool boiling heat transfer analysis of R-134a is carried out on a bare copper surface and over a 50, 100, 150 and 200 nm thickness aluminum nanostructured coated surfaces at 10 °C and 15 °C saturation temperature. Thin film aluminum nanocoating on the polished copper surface is produced by using Thermal Evaporation Physical Vapor Deposition Technique. The coated surfaces are distinguished with the film thickness, roughness by 2D profilometer, imaging by scanning electron microscopy (SEM) and existing of coating material by energy dispersive X-ray spectroscopy (EDX). The effects of thin film aluminum nanocoating over a bare copper surface with increasing heat flux is thoroughly examined. It is observed that maximum enhancement in heat transfer coefficient at 150 nm thickness of thin film aluminum nanocoated surface is about 2.49 times and 2.81 times compared to copper surface over a heat flux range of 6–90 kW m−2 at 10 °C and 15 °C saturation temperatures respectively. Coated surfaces showed a steady increment in heat transfer coefficient values with the increase in the heat flux values. About 26.8% augmentation in heat transfer coefficient of bare copper surface is observed at 15 °C saturation temperature compared to 10 °C saturation temperature. The heat transfer coefficient values of bare copper surface are validated with Rohsenow and Cooper correlations of nucleate boiling and agreed well with measured data with a standard deviation of ±9%, ±13.5% for both correlations at 10 °C and 15 °C saturation temperatures respectively.

Effects of pre-treatments on drying kinetics and energy consumption, heat-mass transfer coefficients, micro-structure of jujube (Zizyphus jujuba L.) fruit

In this study, jujube fruits were investigated effects on drying kinetics and energy consumption (total-specific), heat-mass transfer coefficients, micro-structure of pre-treatments 2% ethyl oleate, 540 W microwave and freeze-thaw + 720 W microwave. Initial moisture of fruit from 0.75 ± 0.2 g (water).g-1 (dry matter) to 0.15 ± 0.1 g (water).g-1 (dry matter). The shortest drying durations were observed in 2% ethyl oleate pre-treated samples. The greatest total power consumption (1.502 kW) was observed in control samples The greatest specific energy consumption (158.98 kW.kg-1) was observed in freeze-thaw pre-treated samples. The effective diffusion values varied between 9.53 x 10-8-5.26 x 10-8 m2.s-1. Lewis and Jena-Das models the best estimated time-dependent moisture ratios in freeze-thaw pre-treated samples. The average speed values was determined between 0.0025-0.0005 (gr db.minute-1) at the beginning of the drying and decreased to about 0.0005 (gr db.minute-1) at the lasting of the drying. The largest mass transfer coefficient value was found depending on the time varied between 1.035 x 10-7-8.256 x 10-12 m.s-1. in the samples dried by dipping into a 2% ethyl oleate solution. The average heat transfer coefficient value is calculated 0.204 W.m2 °C-1. With regard to micro-structure of the dried samples, 2% ethyl oleate pre-treatments yielded the least deformations and had the closest structure to fresh samples.

An experimental study of thermal and hydraulic performance in vehicle cooling system tube with hybrid nanofluids

The aim of work is to observe and analyze the effect of hybrid alumina – oxide and copper oxide nanofluids on the performance with water-based fluid in compact tube. The hybrid nanofluids was synthesized at different volume concentration (0.0 %, 0.40 %, 0.60 % 0.80 %) and Reynolds numbers existing between 4000 and 8000 at 40 °C to 70 °C. The present study helps in analysing the influence of hybrid nanofluids on heat transfer characteristics and finding the effects on heat transfer enhancement as compared to base fluid. Minimum heat transfer coefficient value 310 W/m2K was observed at 0.02 % volume concentration and the Reynolds number was 4000. Pressure drop completely depends upon properties like viscosity, specific heat, thermal conductivity, and Reynolds number. The pressure drop rises with rise in the Reynolds number and increasing volume concentration (v/v) of alumina oxide particles.

Analysis Effects of Average Value of Convective and Evaporative Heat Transfer Coefficient on Solar Cabinet Dryer for Reduction of Mass of Papad

Abstract: In this research paper, the behavior of heat and mass transfer phenomenon during greenhouse papad drying under forced convection mode has been investigated. Various experiments were performed during the month of April 2020 at SRCEM Banmore, morena (26o 34’13” N 78o 10’48” E). Experimental data obtained for forced convection greenhouse drying of papad were used to determine the constants in the Nusselt number expression by using the simple linear regression analysis and, consequently, the values of convective and evaporative heat transfer coefficients were evaluated. The average values of experimental constants C and n were determined as 0.9714 and 0.0129 respectively. The average values of convective and evaporative heat transfer coefficients were determined as 0.0886 W/m2 oC and 6.7583 W/m2 oC respectively. The experimental error in terms of percentage uncertainty was also evaluated. Keywords: Papad, Papad drying, Heat transfer coefficient, Convective, Evaporative, Forced convection greenhouse

Nanotexturing-enhanced heat transfer and interfacial evaporation for energy-efficient solar-thermal water desalination

Seawater desalination based on nano-interface-enhanced evaporation is one of the promising energy-effective solutions to alleviate global clean water scarcity. However, the heat and mass transfer mechanisms from the absorber surface to the surrounding liquid, which is critically significant to improve the evaporation performance in practical applications, are still unclear. Here, we develop an all-carbon hierarchical architecture via engineering of vertically oriented graphenes on a graphite felt monolith, and simultaneously, gain new insights into the effects of the surface morphology on the heat and mass transfer at the absorber/liquid interface by manipulating the graphene texturing from microscale to nanoscale. In the visualization experiments, compared with the microstructured and smooth surfaces, a higher-density bubble distribution with a smaller departure diameter on the nanostructured surface is observed, leading to the significant enhancement of heat transfer through the micro-convection and transient conduction mechanisms. Specifically, the heat transfer coefficient value of the nanostructured surface is more than 1.5 times that of the microstructured surface, thus achieving an increment of ∼5% in freshwater productivity.

Surface wettability effect of plain and plasma-sprayed copper-coated surfaces on CHF enhancement during saturated pool boiling of FC-72

In the current scenario, CHF study is essential for the safe operation of electronics equipment comprising a two-phase heat transfer process. Therefore, the present experimental investigation involves saturated pool boiling and CHF study of FC 72 over a plain stainless steel surface (SS) and microporous copper-coated SS surfaces under atmospheric conditions. Accordingly, three different plasma-sprayed copper-coated surfaces with coating thicknesses of 65 μm, 105 μm, and 145 μm prepared using micro copper particles of size 25–45 μm. The analysis of the results shows that with an increase in heat flux values, the boiling heat transfer coefficient increases over plain as well as coated surfaces. The plasma-spayed copper-coated surfaces with a coating thickness of 65 μm and 105 μm exhibit a higher boiling heat transfer coefficient as than the plain surface. On the other hand, a 145 μm thick coated surface resulted in a comparable boiling heat transfer coefficient with the plain SS surface. Among the three porous-coated surfaces, the boiling heat transfer coefficient decreases continuously from 65 μm to 145 μm of the coated surface. On the contrary, to the observed nucleate boiling behavior, all the porous-coated surfaces show a higher value of CHF than the plain surface, and the CHF value is found to increase continuously from 65 μm to 145 μm of the coated surfaces. The enhancement of CHF values was found to be 66.29%, 69.17%, and 77.75% for a coating thickness of 65 μm, 105 μm, and 145 μm, respectively, compared with the plain surface. The porous coating thickness of 65 μm shows a greater value of heat transfer coefficient than 105 μm and 145 μm whereas 145 μm exhibits a higher value of CHF as than 65 μm and 105 μm.

AERODYNAMICS AND HEAT EXCHANGE OF SINGLE END TUBE DURING EXTERNAL FLOW

In Ukraine, the safety of modern thermal power plants depends on the reliable operation of the equipment installed on them. Unfortunately, the technical condition of the chimneys is not properly maintained. Of course, the modernization of basic equipment (boilers, switching to another type of fuel) leads to a decrease in the temperature of the exhaust gases. An important aspect to maintain the condition of the chimneys is to maintain the moisture of the exhaust gases.
 An important feature of the external flow of chimneys are large Reynolds numbers Re = wd/n, which reach 106 and more. In the thermal calculation only the average heat transfer coefficient on the outer surface of the pipe is usually determined, and the features of aerodynamics and local heat transfer due to the conicity of the pipe are not taken into account.
 The work is devoted to the study of aerodynamics and heat transfer in the air flow of a single conical chimney. The method of computer modeling with numerical integration of equations of motion and energy was used in the research. At the first stage, the single pipe with the uniform flow profile is considered. Further, the influence of the surrounding infrastructure on the aerodynamics and heat transfer of a single conical tube is studied.
 The single conical vertical pipe with 40 m height, 1.7 m diameter at the base and 0.85 m in the mouth was used for the calculation. The computer model was calculated in the ANSYS2020-R1 program. The model is developed in a homogeneous area with the air environment. In order to obtain reliable results, the study was conducted to obtain the optimal set of the grid parameters for the heat transfer conditions. The grids with parameters that affect the distance of the first node from the cylinder wall (options a, b, c, d) and the rate of increase in the size of the elements as they move away from the area of interest (Growth rate GR) were studied. The type of the cylindrical pipe with constant diameter of 1.7 m has been chosen to analyze the sensitivity and to check the grid.
 The turbulence model has been choosen as the following: RNG k-ε model which is common for the tasks of this class, the Enhanced Wall Function, the solution algorithm for the connection of the velocity pressure in stable flows Simplex. It is determined that in case if the distance between the first node from the cylinder wall and the area of interest (Growth rate GR) is more than 8 mm, instability and deviation of the obtained data from the values of the average coefficient of more than 20% appears. As a result of the research, the parameter grid area matching to the “2d” option of table 1 has been selected, i.e.: GR = 1.1, h = 8 mm. In the study of aerodynamics and heat transfer, the conical tube is conventionally divided into 22 sections (with 1 m height each). The case of uniform flow velocity in front of the pipe has been considered.
 As seen, the maximum value of the heat transfer coefficient is in the Zone(21-22). The research shows that oncoming flow velocity of 25 m/s causes the average value of heat transfer coefficient of the conical pipe 62.5 W/m2K, and 61.1 W/m2K according to the known formula . This indicates a small effect of taper on the average heat transfer of the entire pipe.
 In the calculations, three types of surrounding areas are considered: A - open coasts of seas, lakes and reservoirs, rural areas, including buildings less than 10 m high; B - urban areas, forests and other areas, evenly covered with obstacles higher than 10 m; C - urban areas with dense buildings with buildings higher than 25 m. Thus, the wind speed profiles for different types of terrain are nonlinear.
 The wind speed profile in front of the pipe (type of terrain) has a significant effect on the heat transfer coefficient. This confirms the need to take into account the type of terrain and the velocity profile in front of the pipe for local heat transfer.

Experimental Investigation of the Heat Transfer and Pressure Drop inside Tubes and the Shell of a Minichannel Shell and Tube Type Heat Exchanger

This paper presents an experimental study on a shell and tube mini heat exchanger (STMHE). The STMHE consisted of seven tubes in a triangular arrangement, with an 0.8 mm inner diameter and 1.0 mm outer diameter. The heat exchanger shell had an inner diameter of 11 mm, and the heat exchanger had no baffles. For the adopted operating conditions, the Reynolds number on the tube side varied in the range of 300–3000, and 2000–12,000 on the shell side. The aim of this study was to determine pressure drop values during fluid flow and Nusselt number correlations for the heat transfer. A new method based on optimisation was used to derive the equations for calculating the heat transfer coefficients. It allowed the determine of the correlation equations for the heat transfer coefficients simultaneously for both sides of the heat exchanger. The obtained correlations yielded overall heat transfer coefficient values that, in most cases, did not differ by more than ±10% from those determined experimentally. The experimentally determined critical Reynolds number value for the flow inside the tubes was equal to Recr=1160. The Darcy friction factors correlated well with the classical laminar flow correlation and with the Blasius correlation for turbulent flow. The derived correlations for the Nusselt number were best aligned with the Sieder–Tate, Gnielinski, and Kozioł correlations for tube side laminar flow, turbulent flow, and shell flow, respectively. Good agreement between the results obtained using the experimentally derived correlations and the correlations available in the literature confirms the effectiveness of the used optimisation–based method.

Determination of the convective heat transfer constant (c and n) in a solar still

The geometry of a solar still determines the convection constants C and n, which in turn affect the convection heat transfer coefficient’s value and mass. A method for determining the value of convection heat transfer constants C and n has already been developed by the researchers. Therefore, this study aimed to use several methods and theories to find the value of convection heat transfer constants C and n. The results are then compared with the results of the study. The solar still used in this study has one slope. To reduce variables that cannot be controlled, the data collection was conducted indoors using a halogen lamp that can be regulated as a heat source for 24 hours nonstop. The sea surface height in the solar still was maintained at a height of 20 mm, using a height regulator. Temperature was measured using a data logger set to enter data every hour. The desalinised clean water was stored in bottles placed on scales that were recorded every one hour. Room temperature was maintained in the range of 35 to 36 oC. The data in this study were used to calculate the heat transfer constants C and n to obtain the value of the convection heat transfer coefficient and mass calculation. This study compares the calculation models of Tiwari, Dunkle and Power. The following calculation model results: Tiwari model, C = 0.082 and n = 0.612; Dunkle model, C = 0.075 and n = 1/3; Power model, C = 0.815 and n = 0.611. The C and n values obtained with these four approaches reveal that the results from the Power model calculation are the closest to the actual mass, showing a percentage deviation of 1.63%.

Computational and experimental evaluation of heat transfer intensity in channels of complex configuration for gas flow with different levels of turbulence

Disclosure of the physical mechanism of the influence of the turbulence intensity of gas flows on the heat transfer level in pipes of different configurations is an urgent task in the field of heat and power engineering. A brief overview of the literature on this topic is given in the article. A description of the boundary conditions for modeling is presented. The main characteristics of the experimental stand and measuring instruments are described. The purpose of this study is to study the effect of the initial turbulence level of a stationary gas flow on the heat transfer intensity in long pipes with different cross sections. The study is carried out using numerical simulation. The simulation results are qualitatively confirmed using experimental data. The values of the local heat transfer coefficient are shown to increase from 5 to 17% with increasing turbulence intensity (from 2 to 10%) in pipes with different cross sections. The heat transfer intensity in a triangular pipe is found to increase up to 30% compared to a round pipe. It is revealed that there is an up to 15% suppression of heat transfer in a square pipe compared to a round pipe. The data obtained may be useful for the design of flow paths and gas exchange systems for power machines and installations.