Decrease In Heat Transfer Coefficient Research Articles

自動車エンジン排気系流動場を想定した脈動気流における矩形管壁面熱伝達の研究

The airflow in the exhaust pipe of an automobile engine fluctuates periodically in terms of flow velocity and gas temperature, and understanding the complex flow and heat transfer characteristics is extremely important for improving the performance of exhaust systems. In this study, using a horizontal rectangular pipe with a rectangular cross section, we experimentally obtained the time-averaged temperature in the cross section measured by thermocouples and the wall temperature by thermography on the outer wall of the pipe under the pulsation conditions assumed in the actual system, and investigated the effects on the heat transfer characteristics. The influence of flow characteristics on heat transfer characteristics was also investigated by using PIV. The results showed that the heat transfer coefficient peaked at a pulsation frequency of 25 Hz. At higher frequencies, the decrease in velocity amplitude was considered to be responsible for the decrease in heat transfer coefficient. At lower frequencies, the decrease in turbulent energy in the area close to the inner wall was considered to be the cause.

Effects of the casing step size on the tip leakage flow pattern and heat transfer characteristic of turbine blade squealer tip

In this paper, under the GE-E3 turbine stage condition, the effect of the casing step on the heat transfer performance is obtained by solving three-dimensional Reynolds-averaged Navier-Stokes (RANS) equations. The heat transfer characteristics predicted by standard k-ω model are consistent with the experimental data. The simulation results reveal that, the tip heat transfer coefficient decreases and the area of the high heat transfer region is reduced by introducing the casing step structure. Compare to the baseline case, when the casing step height is 1%, 2% and 3% of the blade height, the area-average heat transfer coefficient at the blade tip decreases by 6.1%, 6.7% and 7.0%, respectively. The introduction of casing step structure can eliminate the swirling strength of Scraping Vortex (SV), thus weaken the reattachment of tip leakage flow on the cavity bottom. Under the step vortex effect, the mainstream from the tip pressure side entering the tip cavity is reduced and replaced by entering from the tip suction side. Because of the lower pressure difference between the tip suction side and the tip clearance, the swirling strength of SV is decreased. Arranging a casing step with the same size as the tip clearance upstream the blade can reduce the tip heat transfer performance to the greatest level. The introduction of casing step structure also has a negative effect on the output power and efficiency of the turbine, it is necessary to balance the aerodynamic and heat transfer of the turbine in order to achieve the optimal results.

Numerical Investigations Into the Local Behavior of Condensing R134a Flows in a Horizontal Pipe

Abstract The local characteristics and behavior of condensing R134a flows in a horizontal pipe, at mass fluxes of 100, 200, and 400 kg m−2 s−1, saturation temperature of 40 °C, and mean vapor qualities of 0.25, 0.50, and 0.75, are investigated numerically. The local results demonstrate an increase in condensate film thickness from a particular angular position, which is influenced by the vapor quality of the flow to the bottom of the pipe. The heat transfer coefficient decreases around the pipe circumference from the top to the bottom of the pipe. The results indicate the existence of a particular angular position where the film thickness and the heat transfer coefficient change significantly due to the stratified condensate layer. Furthermore, the velocity profiles of the condensing flows are noted to be asymmetrical due to the stratified condensate layer at the bottom of the pipe. The mean flow velocity and the heat transfer coefficient decrease along the condensation length. The local results demonstrate that the heat transfer coefficient is not merely affected by the condensate film thickness but also by the effective thermal conductivity of the flow. The findings demonstrate the capacity of local measurements in capturing fine features of condensing flows.

ANALISIS KINERJA CONDENSER SHELL AND TUBE UNIT 2 DI PT. PLN (PERSERO) SEKTOR ASAM-ASAM KALIMANTAN SELATAN

Theecondenser is a type of heat exchanger that functions to condense fluid. On steam powermsystems. Thevmain function ofmthe condenser is to convert steam into liquid. The purpose of this study is to determine the value and influence of heat balance, heat transfer coefficient, pressure drop and condenser efficiency.Thewresults offthissstudy indicate that in 2018 thee average heat balance (Q) was obtained at 356,017,533.46 Kj / hour while in 2019 the results of the average heat balance (Q) were 640,293,647,066 Kj / hour, fouling factor was not affect the balance sheet. The average gross heat transfer coefficient (UD) in 2018 amounted to 204,274.25 Kj / hour.m2. C and the average net heat transfer coefficient (UC) was 206,378 Kj / hr.m2. ° C whereas in 2019 the average heat transfer coefficient is obtained by the average gross heat transfer coefficient (UD) of 366,544.07 Kj / jam.m2. ° C and the Clean heat transfer coefficient (UC) is 448,554 Kj / h.m2. ° C.Fouling factor is very influential onnthe heatwtransfer coefficient because the greater the fouling in the tube will result in the inhibition of theeheat transfer rate in the tube, so that the heat transfer coefficient decreases. The pressure drop in 2018 is still within the permissible limits, with an average of 504.28 bars and 2019 of 513.03 bars. The effectiveness of the condenser in 2018 is an average of 23.330 after maintenance has been obtained, the average effectiveness of the condenser in 2019 is 40.743

Onset about non-isothermal flow of Williamson liquid over exponential surface by computing numerical simulation in perspective of Cattaneo Christov heat flux theory

In view of increaing significance of non-isothermal flow of non-Newtonian fluids over exponential surfaces in numerous industrial and technological procedures such as film condensation, extrusion of plastic sheets, crystal growth, cooling process of metallic sheets, design of chemical processing equipment and various heat exchangers, and glass and polymer industries current disquisition is addressed. For comprehensive examination Williamson model expressing the attributes of shear thickening and thinning liquids is taken under consideration. The physical aspects of magnetic field applied in transverse direction to flow is also accounted. Heat transfer aspects are incorporated and analyzed by employing Cattaneo-Christov heat flux model. Mathematical formulation of problem is conceded in the form of PDE’s by implementing boudary layer approach and later on converted into ODE’s with the assistance of transformation procedure. The resulting equations are solved numerically using shooting and Runge–Kutta methods. Impact of involved parameters on flow distributions is displayed through graphs. From the analysis it is inferred that Cattaneo Christov heat flux law exhibits hyperbolic equation which follows the causality principle and make the problem more compatible to real world applications. It is also deduced that magnetic field suppresses the velocity field and associated boundary layer region. Decrease in temperature profile and heat transfer coefficient is found against inciting magnitude of thermal relaxation parameter. Substantial decrease in velocity is found against increasing magnitude of Williamson fluid parameter and magnetic field parameter whereas skin friction coefficient increments. Confirmation about present findings is executed by making comparison with existing literature.

Numerical Analysis of Natural Convection Heat Transfer Inside an Inverted T-Shaped Cavity Filled with Nanofluid

This assessment is centered on the characteristics of natural convection heat transfer of Aluminium Oxide-Air nanofluid inside an inverted T-shaped enclosure with differentially heated sidewalls. The left edges of the enclosed cavity have been treated as a heated wall and are kept at a constant temperature. The right edges are also maintained at a constant temperature but lower than the heated wall. The top and bottom faces of the cavity have been considered adiabatic. The evaluation has been numerically investigated using ANSYS fluent. The effect of different significant parameters like volume fraction of nanoparticles, the shape of the enclosure, and Rayleigh number on the heat transfer characteristics inside an inverted T shape enclosure have been investigated. In this numerical analysis, a series of DNS simulations have been conducted for different Rayleigh numbers in the range of 103 to 106, the volume fraction of particles in the range 0≤ φ ≤0.1, and for the different aspect ratios for the inverted T shape have been conducted. The outcomes of this CFD analysis indicate a remarkable rise in the average heat transfer coefficient with the rising volume fraction of Al2O3 particles in the air. An increase of the average Nusselt number was also observed with the increase of Rayleigh number, but it drops slightly at a higher volume fraction of nanoparticles due to an increase in conductive heat transfer. For Rayleigh numbers ≥ 104, both the average Nusselt number and average heat transfer coefficient decrease up to a certain shape of the cavity aspect ratio. After that cavity aspect ratio, both the parameters value increase. But in the case of Rayleigh number = 103, both of the values decrease with the increase in the cavity aspect ratio.

Investigation of maximum emergency guarantee duration of residential buildings during heating accidents

The maximum emergency guarantee duration (EGDmax) of residential buildings under heating accidents was investigated in this study. DeST was used to simulate the transient room temperature after heating accidents. First, theoretical analyses of the thermal processes of buildings during the heat seasons were conducted. The impact of shape factors, heat transfer coefficients of envelopes, indoor and ambient temperature, and air exchange rate on the EGDmax were investigated, and a corresponding database was developed. Based on this database, a neural network was adopted to predict the EGDmax for different scenarios. The results showed that when the initial indoor temperature increased by 2 °C, the EGDmax of 5 and 0 °C increased by 3–13 and 4–20 h, respectively. With a decrease in the comprehensive heat transfer coefficient of envelopes, the EGDmax of 5 and 0 °C increased by 5–17 and 10–27 h, respectively. The back-propagation neural network model based on the tanh function is the optimal prediction model to predict the EGDmax of 5 and 0 °C. And its root-mean-square errors are approximately 2.20 and 3.14, respectively.

Fluid Flow and Heat Transfer Characteristics Investigation in the Shell Side of the Branch Baffle Heat Exchanger

The branch baffle heat exchanger, being an improved shell-and-tube heat exchanger, for which the flow manner of the shell-side fluid is a mixed flow of oblique flow and local jet. The computational fluid dynamics (CFD) method has been implemented to investigate the fluid pattern and heat transfer performance. The accuracy of the modeling approach has been confirmed by an experimental approach using a Laser Doppler Velocimeter system. Flow field, temperature field, and pressure field are displayed to study the physics behavior of fluid flow and thermal transport. Heat transfer coefficient, pressure drop, and efficiency evaluation criteria are analyzed. In contrast with the shell-and-tube heat exchanger with segmental baffles and shutter baffles, the pressure loss in the proposed heat exchanger with branch baffles has been dramatically improved, accompanied by a slight decrease in heat transfer coefficient under the same volume flow rate. The efficiency evaluation criteria of the heat exchanger with branch baffles are 28%-31%,13.2%-14.1% higher than those with segmental baffles and shutter baffles, respectively. Further analysis in accordance with the field synergy principle illustrates that the velocity and pressure gradients of the heat exchanger with branch baffle have finer field coordination. The current heat exchanger structure provides a reference for the future optimization design to reach energy saving and emission reduction.

A new approach for battery thermal management system design based on Grey Relational Analysis and Latin Hypercube Sampling

A liquid cooling system is an effective type of battery cooling system on which many studies are presented nowadays. In this research, the effects of the mass flow rate and number of channels on the maximum temperature and pressure drop are investigated for multi-channel serpentine cooling plates. A new approach with LHS and GRA is used to obtain the optimum ranges of design parameters to minimize the pressure drop, maximum temperature and to maximize the convective heat transfer coefficient. In this study, the values of the parameters for the numerical modeling are obtained by the experiments. The width and height of the serpentine channel and mass flow rate are chosen as input parameters and the pressure drop, convective heat transfer coefficient and maximum temperature are selected as output parameters. Comparing with the base design, the optimized design provided up to 40.3% decrease in the pressure drop with a penalty of 11.3% decrease in the convective heat transfer coefficient with a slight decrease in the maximum temperature. The proposed approach can be used to design better cooling plates to keep the batteries in safe temperature ranges and to reduce the power consumption by optimizing the pressure drop and maximum temperature values.

Numerical investigation on submerged steam jet condensation in subcooled water flow in a restricted channel with the presence of non-condensable gas

Submerged steam jet in subcooled water flow is widely used in many industrial processes. When non-condensable gas is involved in the steam jet, the mechanisms of heat and mass transfer become more complicated due to the added non-condensable components and the strong effect of water turbulence. In present work, a three-dimensional steady numerical simulation is conducted to study the steam jet condensation in subcooled water flow in a vertical pipe with the presence of non-condensable gas based on inhomogeneous multiphase model. The effects of non-condensable gas on steam plume shapes, distributions of static pressure, velocity, temperature and heat transfer characteristics are investigated. As air contents increases, steam plume transforms from ellipsoidal shape to divergent shape and the peak value of static pressure decreases. The mechanism of the peak phenomenon of static pressure at the end of steam plume is revealed to be the squeezing effect of water flow towards the nozzle axis. Since air mass fraction rises to 1%–9%, the average heat transfer coefficient declines by 24.5%–54.9%. The local heat transfer coefficient is evaluated by using thermal equilibrium model at phase interface. Numerical results show a self-intensifying effect on heat transfer, indicating that the distribution of local heat transfer coefficient is highly similar to that of water turbulent kinetic energy near phase interface, decreasing first and then rising to a high level at the tail of steam plume. The local heat transfer coefficient and steam condensation rate decrease with air concentration and increase with water mass flow rate.

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

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

Numerical Analysis of Heat Transfer and Pressure Drop in Helically Micro-Finned Tubes

In this study, the pressure drop and heat transfer characteristics of smooth tube and internal helically micro-finned tubes with two different fin-to-fin height ratios i.e., equal fin height and alternating fin height, are computationally analysed. The tube with alternating fin height is analysed for proof of concept of pressure drop reduction. A single phase steady turbulent flow model is used with a Reynolds number ranging from 12,000 to 54,000. Water is used as working fluid with inlet temperature of 55 °C and constant wall temperature of 20 °C is applied. Friction factor, heat transfer coefficient, Nusselt number, and Thermal Performance Index are evaluated and analysed. The numerical results are validated by comparison with the experimental and numerical data from literature. The results showed that the thermal performance is enhanced due to helically finned tube for a range of Reynolds numbers, but at the expense of increased pressure drop as compared to a smooth tube. The helically finned tube with alternating fin heights showed a 5% decrease in friction factor and <1% decrease in heat transfer coefficient when compared with the equal fin heights tube, making it a suitable choice for heat transfer applications.

Thermal performance investigation of concentric and eccentric shell and tube heat exchangers with different fin configurations containing phase change material

The aim of this study is to evaluate the combined effect of fin configuration and eccentricity of heat transfer tube on melting behavior of phase change material (PCM) inside the shell and tube heat exchanger (HX). Two unfinned HXs with concentric and eccentric heat transfer tubes and six other HXs with different arrangements of straight and bifurcated fins were fabricated while the total fin mass was kept constant. The shells of heat exchangers were made of transparent Plexiglas and the melting process was photographed to enable the analysis of the solid-liquid interface evolution. Experimental findings of the unfinned HXs showed that applying the eccentric tube HX leads to a 54% reduction in melting time compared to the concentric tube HX. Among all the cases considered, bifurcated fin configuration outperforms the straight fin configuration. The maximum melting time reduction compared to the HX with concentric unfinned tube was 85% obtained by eccentric tube HX with a long upper bifurcated fin and a short lower straight fin. Transient numerical simulations using a control volume approach were carried out for three tube wall temperatures of 75, 85 and 95 °C. Numerical results showed that the bifurcations increase the total heat transfer rate while the convective heat transfer coefficient decreases.

Research on Heat Transfer Enhancement of Vortex-Induced Vibration of Side-by-Side Double Tubes

The fluid-induced vibration heat transfer enhancement technology has great potential, because it can effectively improve the heat transfer efficiency without consuming additional energy. A reasonable design of the elastic heat exchange tube bundle structure is the key to the engineering application of this technology. In this paper, the effects of flow velocity and the center distance on the characteristics of vortex- induced vibration heat transfer in side-by-side double circular tubes are studied. We demonstrate that, when the center distance is no more than (outer diameter), the interference effect of the flowfield around the two tubes is greater, and the amplitude of the double tube is small. However, with the increase of , the flowfield interference decreases. Within the scope of this paper, two modes of vibration are obtained: in-phase vibration and out-of-phase vibration. When and , the fluid vortex-induced elastic supported side-by-side double circular tubes’ vibration can enhance heat transfer. Especially, when , the heat transfer enhancement effect is the strongest, reaching 14.5%. However, when , the heat transfer enhancement effect is not obvious. The research on the heat transfer characteristics of fluid flowing around the static double tube shows that when the distance is small, the flow velocity around the static tubes is larger, resulting in a higher surface average heat transfer coefficient, and as the distance increases, the heat transfer coefficient decreases.

Experimental Study on Behavior of Coolants, Particularly the Oil-Cooling Method, in Electric Vehicle Motors Using Hairpin Winding

This paper analyzes the characteristics of oil behavior in the oil-cooling of motors with hairpin winding to understand how to maximize cooling performance. The oil cooling is performed by directly spraying oil onto the motor components. The results show that as the temperature of the oil increases, the viscosity decreases, and the oil film is formed more evenly; however, oil splashing also increases. Similarly, as the flow rate increases, oil splashing also increases, but the amount of oil forming the oil film increases. However, the oil film is not affected by the rotor’s rotation. In contrast, the immersed oil is found to be closely related to the rotor’s rotation. As the rotational speed increases, the immersion oil is mixed with the air, and oil churning occurs. The mixing phenomenon increases as the temperature and flow rate of the oil increases. The higher the oil level, the greater the oil churning. As the oil is mixed with air, the heat transfer coefficient decreases, which adversely affects the thermal management of the motor. As a result, when considering the oil film and the immersion oil, the optimal oil temperature, flow rate, and oil level are at 60 °C, 0.140 kg/s, and 85 mm, respectively. The results of this paper give important information about EV motor cooling and can contribute to the development of high-performance motors.

Effect of concentration and sedimentation on boiling heat transfer coefficient of GNPs-SiO2/deionized water hybrid Nanofluid: An experimental investigation

One of the most investigated and broadly used heat transfer processes in the industry is the boiling heat transfer. In this research, an experimental investigation concerning the boiling heat transfer of the silicon oxide-graphene hybrid nanofluid in the deionized water, as the basefluid, was done. The nanofluid employed in this study was produced by a two-step method having very high stability. For the deionized water, the repeatability and accuracy of the experimental setup were performed four times, which has a perfect overlap to the available results in research history. The results showed that the boiling heat transfer coefficient (BHTC) of the nanofluid decreases by increasing concentration. This heat transfer coefficient decrease was observed in high heat fluxes. The surface roughness of the boiling changes by increasing concentration. Moreover, through increasing the concentration of the nanofluid, its surface tension also enhances, which has a great influence on the boiling process. Furthermore, it was reported that the boiling heat transfer is inversely related to the surface tension so that it decreases by increasing the surface tension. BHTC of DW, 0.1 vol%, and 0.8 vol% nanofluid was around 18.2, 11.6, 11.6 kW/m2.K at about 140 kW/m2 Heat Flux that enhanced to 44.2, 31.9, and 29.9 kW/m2.K at about 700 kW/m2 Heat Flux, respectively. The highest uncertainties of 3.4% and 7.5% were calculated for the heat flux and BHTC.

Volumetric heat transfer coefficient in spray drying of soymilk powder

In this study, an empirical equation of the volumetric heat transfer coefficient in spray-drying of soymilk powders was developed. Response Surface Methodology RSM was employed to examine the effects of feed concentration and inlet air temperature on the volumetric heat transfer coefficient in the drying chamber. Overall, results show that the concentration of feed solution had a greater effect than the inlet air temperature. An increase in the feed concentration led to a sharp decrease in the volumetric heat transfer coefficient in the drying chamber, whereas the volumetric heat transfer coefficient increased slightly with rising the inlet air temperature of spray dryer. Highlights Volumetric heat transfer coefficient of spray dryer was compared to other dryers. Empirical equation of the volumetric heat transfer coefficient was developed. Response Surface has been a relevant methodology to examine the spray drying conditions Concentration of feed solution had a greater effect than the inlet air temperature.

Nucleate boiling heat transfer of R-134a and R-134a/POE lubricant mixtures on smooth tube

The present study examines the influence of polyolester lubricant oil (POEA-68 and POEA-170) with R-134a on the nucleate pool boiling performance for a horizontal smooth tube. The heat flux ranges from 10 kW/m2 to 90 kW/m2 and the lubricant mass fraction varied from 1% to 10% at a saturation temperature of 10 °C, 0 °C and, −6 °C, respectively. The experimental results show that the heat transfer coefficient (HTC) of the pure refrigerant is increased with an increase in saturation temperature. But the presence of lubricant oil alters the HTC considerably. The addition of 3% POEA-68 lubricant shows the highest 29.3% HTC enhancement relative to pure refrigerant at a supplied heat flux of 70 kW/m2 and at a saturation temperature of 0 °C. Whereas for R-134a/POEA-170 lubricant mixture at 3% of mass fraction, the highest enhancement in HTC was around 26% at a supplied heat flux of 70 kW/m2 and −6 °C saturation temperature. The bubble size is reduced and the bubble density is increased appreciably with the addition of POE oil. It was observed that the HTC of lubricant-refrigerant mixtures differs appreciably with the variation of lubricant oi mass fraction (ω). The HTC first decreases at a very low mass fraction (ω≤1%), followed by an appreciable increase in the range of 1% ≤ ω ≤ 3%, and finally reveals a continuous decline for the higher mass fraction. The dramatic decrease in HTC for lubricant-refrigerant mixture is associated with mass transfer resistance and a significant increase in surface tension.

Simulation of forced convective heat transfer in Kelvin cells with optimized skeletons

The optimization of pore structure for metal foam is a feasible method for reducing the pressure drop and improving the overall heat transfer performance, especially at high velocity. In this regard, the optimized Kelvin cell with elliptical skeletons with a cross-section ratio of the long axis to the short axis (b/a) of 1.0, 1.4, and 2.0 are considered to evaluate the effects of b/a on the pressure drop and the heat transfer coefficient (HTC). The results indicate that both the pressure drop and HTC decrease with an increase in b/a. However, the pressure drop reduces more significantly. For instance, as b/a increases from 1.0 to 2.0, the pressure drop and the volumetric HTC at 90 m/s decrease by 98.5% and 6.3%, respectively. Therefore, the value of the volumetric area goodness factor (which considers both the effect of heat transfer coefficient and pressure drop on the overall heat transfer performance) jv/fof the sample with b/a = 2.0 is 86.7% higher than the sample with b/a = 1.0 at 90 m/s. As the velocity increases, the effect of b/a on the overall heat transfer performance increases. Compared with the cylindrical skeleton velocity distribution and pressure drop, the elliptical skeletons lead to a more uniform velocity distribution and reduce pressure drop significantly; nevertheless, the temperature distributions barely change. Moreover, the elliptical skeleton increases the specific surface area. Therefore, the jv/fof the elliptical skeleton significantly improves. This study provides a new direction for the design of novel metal foams with excellent overall heat transfer performance for heat transfer devices.

PREDICTING OF STEAM CONDENSATION HEAT TRANSFER COEFFICIENT IN HORIZONTAL FLATTENED TUBE

The heat transfer coefficient of steam condensation has a significant role in the performance of air-cooled heat exchangers. The purpose of this work is to predict the local/average local steam condensation heat transfer coefficient inside the horizontal flattened tube under vacuum conditions using numerous correlations that were developed by some researches which have been conducted under specified conditions. The results from these correlations have been compared with experimental data of Davies, therefore more investigate for the values are necessary to improve or/and validate the existing correlations. The effect of such parameters like the uniform heat flux and saturation temperature also have been studied on the local steam condensation heat transfer coefficient as the results show that the heat transfer coefficient decrease as the heat flux increase, while it increases as the steam saturated temperature increase.