Total Heat Transfer Rate Research Articles

Thermal-hydraulic characteristics of printed circuit heat exchanger used for floating natural gas liquefaction

Printed circuit heat exchanger is considered a promising candidate for floating natural gas liquefaction because of its high compactness, high pressure resistance, and high efficiency. In this work, a numerical model is established to study the thermal-hydraulic characteristics of transcritical natural gas in the printed circuit heat exchanger in rolling conditions. The results show that the rolling condition has few effect on the total heat transfer rate, but significantly affects the local friction factor and heat transfer characteristics. The rolling condition enhances the local heat transfer performance of transcritical natural gas, but also increases the pressure drop, in which the influences of rolling are the greatest at 1/4 and 3/4 of the rolling period. The tangential force and Coriolis force caused by the rolling motion increase the heat transfer of the natural gas. In the subcritical zone, where the rolling condition has the greatest influence on the heat transfer of the natural gas, the maximum instantaneous Nusselt number is increased by 40%. The rolling condition shows minor effect on the pseudocritical zone, in which the maximum instantaneous Nusselt number is increased by 15%. The rolling condition has almost no effect on the heat transfer performance of natural gas in the supercritical zone. The effect of the rolling condition on the flow characteristics of natural gas is the greatest in the pseudocritical zone, where the friction factor and local pressure drop are increased by approximately 30% compared to those in the steady condition.

Modeling of an air cooler with finned heat transfer tube banks using the RELAP5-3D code

The present paper proposes a calculation model of an air cooler using the RELAP5-3D code. Up to now, the modeling of an air cooler using RELAP5-3D was not described in detail, and there was a problem when calculating the decay heat removal system with the air cooler. Above all, the study of the heat transfer model for an air cooler with finned heat transfer tube banks is few. Since a decay heat removal system with air coolers is essential in liquid metal cooled fast nuclear reactors, it is anticipated how to model the air cooler using implemented heat transfer models in the RELAP5-3D code. The finned heat transfer tube is simulated with multiple bare tubes which have the equivalent heat transfer characteristics considering fin efficiency. Two kinds of simulations have been compared to the measured results of the “Monju” reactor under the forced and natural convection conditions in order to confirm the applicability of the proposed model. The simulation result under the forced convection condition is in good agreement with the test result except the operation mode under the natural convection. Since the simulation result under the natural convection condition overpredicts the test result, the surface area is lessened in order to decrease the heat transfer. As a result, good agreement has been obtained between the simulation and the test result under the natural convection condition. Sensitivity analysis has been performed for the cases where the outside diameter and the number of the simulated heat transfer tubes, and number of divisions of the calculation meshes are changed. It has been confirmed that there is almost no change in the total heat transfer rate even if various diameter tubes are used and the number of divisions is changed. The number of heat transfer tubes has a non-linear sensitivity to the removal heat rate.

Natural Convection and Radiation in a Cavity with a Partially Heated Cylinder

The interaction effects of surface radiation with natural convection of a transparent medium in an asymmetrically cooled square cavity including a partially heated inner circular cylinder have been numerically simulated. In this context the finite volume method and the discrete ordinates method are incorporated to simulate radiation problems. The flow inside the cavity is driven by the heated part of the circular cylinder maintained at high temperature . The cavity is filled with air () cooled by a single vertical wall maintained at a fixed cold temperature . The other portion of the inner circular cylinder and the remaining walls of the enclosure are insulated. The hydrodynamic and thermal behavior of the fluid and the convective and radiative heat transfer are investigated for Rayleigh number ranging from to , the common emissivity of the inner and outer surfaces, the angular size of the circular heater, and its angular position . The numerical work is carried out using an in-house computational fluid dynamics code. The obtained results showed that the role of radiation is well stronger than convection in determining the total heat transfer rate when the position of the heated portion is changed.

THE EFFECT OF ASPECT RATIO OF ELLIPTICAL COOLING HOLES ON THERMAL CHARACTERISTICS OF GAS TURBINE BLADE

In advanced gas turbine, the improvement of the thermal efficiency and power output is required. Among these improvements, which still getting attention considerably, increasing turbine inlet temperature that may exceed the melting point of the blade material and to avoid that internal blade cooling technique is incorporated. The common traditional cross-sectional area of the cooling channels is circular. In this paper, a modification regarding the shape of cooling hole is suggested to enhance the heat transfer between the coolant in cooling serpentine and hot gases from the combustion chamber and thus minimizing the severe temperature usually occurs in the gas turbine blade especially at first row of the rotor. The proposed shape is elliptical and to discuss different scenarios of cooling schemes, the aspect ratio of ellipse axes and the distribution of whole holes are also analyzed to figure out the best results in terms of temperature distribution, heat transferred and cooling effectiveness of each model. The three new models consist of cooling holes of (5, 6 and 7) located at the center of the curvature line of blade. Results were compared with those obtained using the model of the GE company (model-1) which is already installed in (AL-DIWANIAH GAS TURBINE POWER PLANT). It were analyzed on the basis of real data, such as geometry profile, total cooling area, and boundary conditions. Results have been discussed and it is found that model-4 is the optimum solution, because of maximum total heat transfer rate and reduction in the blade trailing edge temperature about 24% are attained in this model compared to model-1, and it is found increase number of cooling holes in the blade leads to lowers the blade trailing edge temperature, this depends on the design location of this holes on the blade surface. Meanwhile, the optimum ratio of the model-4 in the reduction of maximum temperature is (a / b = 2) by the dimensions of (a = 2 & b = 1). Steady state thermal analysis is carried out using CFD, Inconel 718 alloy selection as material and air is used as a coolant as the real condition of.

Utilization of waste heat from commercial kitchen exhaust for water heating and dish drying

In the cooking process, abundant waste heat is generated and usually discharged out of a room along with the exhaust air directly. Obviously, recovering heat from the exhaust air by a heat pump is a promising technology in net-zero energy buildings, which can improve energy utilization efficiency and reduce exhaust emissions. A water-heating and dish-drying system based on a heat recovery unit was proposed in the current work to recover heat from the exhaust air of a commercial kitchen. The influences of key factors, including terminal functions, cooking methods, exhaust air modes and drying items, on the operating performance of this system were investigated. Results indicate that the heat transfer rates of water-heating and dish-drying are 7.17 kW and 0.40 kW respectively, accounting for 95% and 5% of the total heat transfer of the system. For these two functions, the total heat transfer rate and COP of this system are almost the same, leading to no influence on the system's performance, except for the changes in the heat distribution. Furthermore, due to differences in the temperature, moisture content, and stability of the smoke produced by different cooking methods, the proportions of the sensible heat and latent heat are different when the smoke exchanges heat on the evaporator. However, the total heat of the evaporate side has little change by different cooking methods.

A study on impingement heat transfer characteristics of partially premixed flames on double-concentric-pipe burner

This paper presents an experimental study of free flame patterns, impinging flame structures and flame impingement heat transfer characteristics of a partially premixed flame. The partially premixed flame is produced on a double-concentric-pipe burner, with both flow strength and mixedness of its inner and outer jets controlled by digital flowmeters. The aim of the study is, through examination of the effects of inner and outer jet Reynolds numbers (Rei, Reo), equivalence ratios (Φi, Φo) and nozzle-to-plate distance (H), to understand the heat transfer behaviors of such partially premixed flame and further identify the best impinging flame to achieve higher energy usage efficiency.A total of twelve different free flame patterns are observed by varying Rei-Reo-Φi-Φo, and W-shaped flame is the most representative flame under the conditions tested. The partially premixed dual flame is higher in stability than any single component of it, i.e. Bunsen flame or annulus flame. Further, leaner flammability limit is achieved by the dual flame due to mutual interaction which effectively supports leaner inner flame. Both inner and outer flames influence the radial heat flux distribution of W flame, while the contribution of inner flame becomes significant only when the difference in two flame heights is large. The outer flame globally dominates the total heat transfer rate, leading to general decay with increasing H. Comparison of W flame with single Bunsen and annulus flame shows that W flame has the favorable structure for higher energy efficiency due to double-pipe configuration and mutual flame interaction.

Comparative study on the thermal-hydraulic performance of a shell and plate particle-SCO2 moving packed bed heat exchanger with various channel configurations

This article proposes a predictive static model to evaluate the thermal-hydraulic performance of a shell and plate particle-SCO2 moving packed bed heat exchanger, with consideration of pressure drop, radiation and property variations. The results reveal that convection resistance on the SCO2 straight channel side occupies a large part of total thermal resistance in most design and off-design cases. A three-dimensional model inheriting conclusions from the static model is raised, which demonstrates that extremely short entrance region in the x-y plane hinders the overall heat transfer behavior of the SCO2 straight channel. Deservedly, suppressions of the development of thermal layers in the y direction will contribute to improving the thermal-hydraulic performance of the shell and plate particle-SCO2 moving packed bed heat exchanger. Four different fin configurations on SCO2 channel side are presented, analyzed and compared via the static model at the design-point case. Offset rectangular fins configuration behaves the best heat transfer enhancement in total heat transfer rate and overall heat transfer coefficient when compared with the original channel, followed by zigzag fins configuration, offset airfoil fins configuration and s-shaped fins configuration. Zigzag fins-modified channel expresses the worst hydraulic behavior, of which the pressure drop is nearly four times that of original straight channel. The pressure drop of channel with s-shaped fins nearly equals to that of the unmodified channel. Introduction of fins on SCO2 channel side reduces the ratio of convection resistance as a result of apparently increased SCO2 convection coefficients.

Investigation on dynamic behaviour of condensation heat transfer in indirective evaporative cooler

Indirect evaporative cooling is a rapidly developing air-handling technology and has great application potentials for energy recovery in hot and humid regions. The condensation in dry channels of an indirect evaporative cooler (IEC) occurs when the dew point temperature of outdoor air is high. However, the dynamic heat transfer performance of an IEC during the process of condensation evolution was rarely discussed. This paper aims to experimentally investigate the dynamic behaviour of primary air condensation and its effects on the convective and total heat transfer rate of an IEC. A transparent cover plate is placed outside an aluminium heat exchanger plate to visualize the evolution process of droplets retained on the surface. The droplets' falling frequency was recorded and the accumulated condensate mass was correlated based on an analytical method taking into account the contact angle and droplet volume. Results showed that the dynamic dehumidification performance can pose great influences on the convective heat transfer in IEC. The gradually diminished dropwise regions and increase of filmwise regions deteriorate the wet-bulb effectiveness of IEC by 14.8%. The convective heat flux keeps decreasing with the accumulation of condensate retention until a dynamic equilibrium is achieved between the retained and falling droplets.

Numerical and experimental study on thermal-hydraulic behaviors in the secondary side of passive residual heat removal heat exchanger

Passive residual heat removal heat exchanger (PRHR HX), as a key component of the passive residual heat removal system (PRHRS) in the advanced reactors of CAP1400 and AP1000, should have sufficient heat transfer capacity to remove core decay heat during the accidents. In this paper, experiments and numerical simulations are constructed to explore the secondary side flow and heat transfer characteristics of a miniaturized PRHR HX that comprises 6 × 7 C-shaped tubes located in the center of a rectangular tank. In the three-dimensional (3D) numerical simulation, the C-shaped tubes are simplified as porous media and the drift flux model with the distributed resistance method is employed to simulate the two-phase flow in the secondary side of PRHR HX. The heat removal rate and mass transfer rate in the tank are estimated using empirical correlations. In the test, both the primary side and secondary side temperatures are measured. The bubbles generated on the tube outer walls are recorded using a high-speed camera. The local outer wall temperatures, total heat transfer rate and local water temperatures are all compared with experimental values to verify the simulation. The maximum deviation of the calculated heat transfer rate is less than 4.5%. The details about the secondary side flow conditions and bubble behaviors are analyzed.

Intelligent optimization design of shell and helically coiled tube heat exchanger based on genetic algorithm

The intelligent optimization design of a helically coiled tube heat exchanger is proposed. The structural design, meshing, and numerical calculations are integrated into the genetic algorithm to perform intelligent optimization in selecting the structural and thermodynamic parameters. Compared with the experimental results, the heat flux and heat transfer rate of the heat exchanger with the optimal structure increase by 110% and 101%, respectively, which can be proved by the decrease of the average intersection angle between the velocity vector and temperature gradient on both the shell side and tube side. Considering the pressure drop constraint, the maximum heat flux increases by 12% compared with the value obtained from the optimization criterion when the total heat transfer rate is maximized, indicating its potentials in reducing the financial cost. Finally, this method provides an automatic solution to the optimization design of a diversity of heat exchangers.

DNS of droplet impact on a solid particle: Effect of wettability on solid conjugate heat transfer

Heat transfer between solid particles and liquid droplets is a key phenomenon encountered in many industrial applications. We investigate the fluid dynamics and heat transfer between a liquid droplet and a static solid particle by means of direct numerical simulations. The presence of the solid particle is represented through a sharp interface immersed boundary method (IBM) on a Cartesian computational grid, whereas the motion of the gas-liquid interface is tracked by a mass conservative volume of fluid method (VOF). To account for the thermal coupling between the gas and the liquid, and more specifically the conjugate heat transfer between the fluid and the solid, a second order accurate diffused interface formulation is proposed for the conduction term in the energy equation. A detailed validation of the method is also presented. We have studied the effect of wettability and solid conductivity both of which have been found to have significant effect on the total heat transfer rate.

Natural Convection and Surface Radiation Heat Transfer in a Square Cavity with an Inner Wavy Body

An in-house CFD code has been developed to address the coupling between natural convection and surface radiation in irregular geometries encountered in cooling electronic components. In the current numerical study, the discrete ordinate methods with the concept of blocked-off region are used in order to investigate the conjugate natural convection and surface radiation in an enclosure containing a wavy inner circular cylinder. The validity of the numerical code is ascertained by comparing our results with previously published results dealing with the limiting case of a smooth circular cylinder enclosed in a cavity. Extensive computations have been performed to study in detail the effect of the Rayleigh number ( $$10^{3} \le {\text{Ra}} \le 10^{7}$$ ), the inner and outer surface emissivities ( $$0 \le \upvarepsilon_{{{\text{E}}\,{\text{or I}}}} \le 1$$ ), the undulations number ( $$4 \le {\text{N}} \le 1 2$$ , their amplitude ( $$0 \le {\upalpha} \le 1$$ ) and the average radius of the wavy circular cylinder ( $$0.05 \le {\text{R}}_{ 0} \le 0.25$$ ) on the flow structure as well as on the convective and radiative heat transfer characteristics. The obtained results showed that the amplitude and the average radius have more significant effect on the total heat transfer rate than the undulations number of the wavy wall.

Comparison of Heat Transfer Performance for Condensation in Corrugated Low Finned Tube and Smooth Tube with Air

The existence of air served as non-condensable gas makes it difficult to design effective titanium condenser in PTA plant. In this paper, the heat transfer characteristic for condensation inside corrugated low finned tube and smooth tube in presence of air was compared. The local heat transfer performance was studied by comparing bulk temperature and local heat transfer rate. The effect of non-condensable gas mass fraction on overall heat transfer performance was also analysed. The results indicated that both the bulk temperature and the local heat transfer rate decreased along the axial direction due to the reduction of steam partial pressure and temperature difference. The average heat transfer coefficient and total heat transfer rate decreased with the increase of inlet non-condensable gas mass fraction. And the corrugated low finned tube could reduce the impact of non-condensable gas by reducing the thickness of gas film and condensate film.

Lehimli Plakalı Isı Değiştiricisi Isı Transferinin Sayısal Analizi

In this paper, we investigated the heat transfer characteristics of a brazed plate heat exchanger by using Computational Fluid Dynamics (CFD). Heat exchangers were generally analyzed considering both rating and sizing parameters. We just examined the rating parameters such as flow direction, mass flow rate, and the inlet temperature value of hot water on the thermal performance of a brazed plate heat exchanger, numerically. We also employed experiments for getting comparative results and each experiment was performed under the same conditions selected in the numerical simulations. Theoretical calculations were achieved by using the Logarithmic Mean Temperature Difference (LMTD) method for a single-phase flow and the thermal efficiency of the heat exchanger was evaluated for different conditions. The maximum total heat transfer rate was calculated about 3.1 kW for experimental study whereas this value was lower than 3 kW for numerical study in counter flow conditions. Another important result was that the effectiveness values of the heat exchanger were calculated higher under counter flow conditions as expected. The numerical results were in good agreement with the experimental data used in the study.

Constructal optimization for an organic fluid shell-and-tube heat exchanger based on entransy theory

Based on constructal theory and entransy theory, constructal optimization of a shell-and-tube heat exchanger (STHE) for the organic fluid evaporation process is performed using a complex function as an optimization objective. The complex function is composed of the entransy dissipation rate (EDR) caused by heat transfer and the total pumping power. The total heat transfer rate and heat transfer area are taken as constraints in the optimization. The minimum complex function and optimal external diameter of the heat transfer tube can then be obtained. The results show that, compared to the initial design construct, the optimal construct of the STHE reduces the EDR caused by the heat transfer and complex function by 9.54% and 1.56%, respectively, and increases the total pumping power by 22.40%. This illustrates that an overall performance improvement of the STHE can be realized by substantially reducing the fluid flow performance and significantly decreasing the heat transfer irreversibility. The overall performance of the STHE can be further improved by increasing the total heat transfer area and the mass flow rate of the working fluid and decreasing the inlet temperature of the working fluid, as well as choosing an appropriate total number of heat transfer tubes and an optimal mass flow rate of the hot water. The optimal constructs of the STHE derived from the minimizations of the two complex functions, composed of the EDR and total pumping power, heat transfer rate and total pumping power, are obviously different. New guidelines for the optimal structure designs of STHEs are provided by introducing entransy theory into constructal optimizations of organic fluid STHEs.

Investigation on the ash deposition of a radiant syngas cooler using critical velocity model

Coal gasification technology is a very important industrial technology, in which the Radiant Syngas Cooler (RSC) plays a pivotal role in the heat storage and heat recovering process. Ash deposit in RSC is a critical factor which may seriously affect the performance of RSC or even result in a shutdown. Therefore, conducting research on the deposit process of RSC is extremely worthwhile. In this study, we create an RSC model with the application of an optimized Critical Velocity Model (CVM) and perform simulations using the CFD method. With the simulating method, we conduct a series of analysis to obtain a more detailed description of the ash distribution and heat transfer in the RSC. Research shows that the CVM proposed in this paper can be precisely applied to the heat transfer process in a RSC, and not only can this method obtain the distribution of ash deposit similar to other methods but also determine the conversion of the ash deposit thickness to heat resistance, which makes the forecast of the overall heat exchange more accurate. According to the study, the ash thickness changes with the vertical height of RSC. For the division wall and panels, the ash thickness increased from the bottom, and then reached a maximum value at about 2/3 of total height. Under the influence of ash deposition, the total heat transfer rate of RSC decreased by 10.4%, and the outlet temperature of syngas increased by 5.7%. This result can provide a reference for the predictive analysis of RSC.

Fork-shaped constructal fin array design a better alternative for heat and mass transfer augmentation under dry, partially wet and fully wet conditions

In actual practical situations, the evaporator fins of refrigeration and air-conditioning apparatuses may operate under dry, fully wet or partially wet conditions. Thus in the present study, the optimum design of an array of constructal fork shaped fins operating under partially wet conditions has been determined by using a linear variation of the humidity ratio of saturated air with the corresponding fin surface temperature. The point of separation between the dry and wet portions may be located either in the stem or branch part of the fin and hence, two cases having different governing equations and boundary conditions are analysed in the current study. The optimisation study is done by maximising the total heat transfer rate of the fin array and un-finned surface and the constraints are imposed in such a way that fin material limitations, minimum fin gap requirements as well as space limitations in the radial direction are taken into account. For obtaining the optimum condition, a swarm based meta heuristic algorithm called Firefly algorithm has been employed. Arrays of fork-shaped fin having two and three numbers of branches are analysed in the present study and a comparative study is done with the rectangular fin array. For more compact evaporators, it would be more preferable to select the fin array that results in larger optimum heat transfer rate as well as that having a smaller total length of each fin. An important finding is that the array of fork–shaped fins with two branches give higher optimum heat transfer rate than that from the rectangular fin array in all the cases of different operating conditions. However, no advantage with respect to either higher optimum heat transfer rate or smaller fin length is obtained when the number of branches of fork shaped fins is increased from two to three.

Coupled simulation and validation of a utility-scale pulverized coal-fired boiler radiant final-stage superheater

The paper presents a coupled modelling methodology that employs a three-dimensional finite volume method computational fluid dynamics model to solve the boiler flue gas side, and a 1D FVM network approach to model the steam side flow and heat transfer of a final-stage radiant superheater. The CFD model includes the necessary upstream components that affect the thermal performance of the superheater, including combustion and heat transfer in the furnace and platen superheater. The methodology is applied to a 620MWe drum-type boiler of which the superheater was instrumented with thermocouples measuring the outlet steam temperatures on the different elements. This was augmented with boiler process data that was extracted from the distributed control system (DCS) for steady-state operation at 99% and 65% boiler loads.The results show that the approach can capture the details of the temperature maldistribution between legs in the same heat exchanger, between elements in the same leg and between the inner and outer loops of the same element for different load cases. It also captures the steam flow maldistribution between the inner and outer loops. On the flue gas side the results show that for the part-load case, the high gas temperature and velocity zones are less uniformly distributed compared to the full load case and also less symmetric than in the full load case.The calculated total heat transfer rates for the evaporator, platen superheater and final superheater compare well with the experimental values for both the 99% and 65% maximum continuous rating load cases. This shows that the coupled simulation models can capture the overall thermal performance of the three heat exchangers modelled here with good accuracy.

Study of the reactor temperature effect on H[formula omitted] production from steam decomposition using DBD plasma

Heat transfer data for the effect of the reactor and surrounding temperature on the hydrogen production from water vapour decomposition using Dielectric-Barrier Discharge (DBD) plasma are reported. The water vapour was injected into the plate micro channel reactor (PMCR) where the PMCR and its surrounding temperatures were raised using an insulated constant temperature heater type in a range of 20–130°C. This experiment was run at the following steam inlet conditions: pressure 100 kPa, temperature 300°C and flow rates of 100–200 ml/h, while the plasma applied voltage was in a range of 14–18 kV. In particular, the concentrations of hydrogen and oxygen species were analysed using gas phase chromatography (GC). It was observed that the hydrogen concentration percent increased with the reactor and surrounding temperature increased at all tested steam flow rates. Also, it was seen that the maximum obtained H2 concentrations were at high reactor temperature 130°C and plasma applied voltage of 18 kV. The effect of reactor temperature on the total heat transfer resistances at different steam flow rates was investigated. The trend behaviour of these resistances reveals interesting information about how these resistances interact and distribute according to the reactor heating temperature. Also, it was found that the total heat transfer rate decreased with the reactor temperature increased while the overall heat transfer rate increased with plasma voltage and reactor heating temperature. It can be concluded that the DBD plasma effect improved by decreasing the temperature difference between input steam and the PMCR temperatures. Further, it was clear that the steam dissociation processes not mainly depending on the plasma applied voltage but also the reactor heating temperature.

Thermal optimization of shell and tube heat exchanger using porous baffles

In this study, total heat transfer rate and pressure drop along a shell and tube heat exchanger (STHX) with 6 porous baffles are numerically investigated. To study the impacts of segmental porous baffles, three values for the permeability (10−9 m2, 10−12 m2, and 10−15 m2), porosity (0.2, 0.5, and 0.8), and baffle cut (25%, 35%, 50%) were considered, and the output parameters were calculated. Low baffle cuts provided the highest heat transfer; however, it generated a considerable amount of pressure drop as well. Although the porosity of 0.2 was superior in terms of heat transfer, higher pressure drop at lower baffle cut is the obstacle to consider it as the optimum value. The data was then utilized to train an Artificial Neural Network (ANN) to characterize the STHX and perform the sensitivity analysis. Baffle cut had the highest impact on the heat transfer as well as pressure drop while the porosity had the least in both by having 5% contribution. Finally, using genetic algorithm (GA), the optimum values for permeability, porosity, and baffle cut are calculated to gain the maximum heat transfer and minimum pressure drop in the system.