Total Heat Transfer Surface Area Research Articles

Heat transfer analysis of subcooled flow boiling in copper foam helical coiled heat exchanger – A pore-scale numerical study

High performance thermal energy systems are crucial in fulfilling the growing demand of thermal energy. Consequently, numerous heat transfer enhancements have been proposed. Some of the promising methods are boiling flow heat transfer, helical coiled tube and metal foam insert. Although the performance of each strategy has been extensively studied, based on our literature review, the effectiveness of combining these strategies has not been investigated. This missing information may impede further development of efficient heat transfer systems. This study is therefore performed to investigate subcooled flow boiling heat transfer in helical tube with copper foam insert. A high-resolution three-dimensional pore scale model that considers the shape, porosity and pore density of the metal foam is developed and validated against the published experimental results. Using the model, the effect of inlet temperature, mass flux, wall heat flux, porosity, pore density and half-filled metal foam are studied. The results suggest that operating parameters have mild effects on the heat transfer performance. In contrast, porosity and pore density show significant influence in heat transfer coefficient and pressure drop. It has also been found that total heat transfer surface area and base area are better in characterizing heat transfer in pore-scale level than porosity and pore density at pore-scale level. Moreover, implementing metal foam at outer half of the tube is observed to offer slightly better heat transfer than implementing it at the inner side by around 4.8%. Overall, this study indicates that helical metal foam tubes offer the best performance at mass flux of 200 kg/m2∙s with performance index of 0.72, while straight tubes with 0.90 porosity has the highest performance index of 0.66. Empirical correlations to relate dimensionless parameters to the Nusselt number and friction factor is generated. Finally, the model developed in this study can serve to provide guidelines for the upcoming studies in boiling in metal foam structure and assist in designing metal foam heat exchangers.

Experimental comparison on energy consumption and heat transfer performance of corrugated H-type and L-type brazed plate heat exchangers

Global energy agencies and commissions highlight the importance of energy efficient and high performance systems to maintain a sustainable environment. At this point, brazed plate heat exchangers provide enhanced thermal performance with design compactness and reduce high energy consumption caused by the pumping power. This experimental work uncovers the thermal and hydraulic performance of the corrugated H-type and L-type brazed plate heat exchangers. The selected heat exchanger systems satisfy two main criteria: identical total heat transfer surface area and total volume of fluid within the systems. The experiments are conducted between 50 L/h and 200 L/h flow rates, and the temperature levels vary in a range of 10 °C to 50 °C. The uncertainty analysis for the experimental equipment and components have been documented. Furthermore, the experiments have been repeated three-times to eliminate the measurement errors and discrepancies. The findings indicate that heat transfer rate experienced by the H-type BPHE is higher than the L-type topology for the entire volumetric flow rates, and heat transfer enhancement reaches up to 7.4%. In contrary to the thermal performance, the L-type brazed plate heat exchanger yields lower energy consumption than the H-type system (except 50 L/h flow rate). The experimental findings show that total energy consumption between 100 L/h and 200 L/h flow rates decreases up to 10.5% via the L-type corrugated topology.

Evaluation of bed-to-tube heat transfer in a fluidized bed heat exchanger in a 75 MWth CFB boiler for municipal solid waste fuels

Bed-to-tube heat transfer has been investigated for a tertiary superheater in a 75 MWth Circulating Fluidized Bed (CFB) boiler in Norrköping, Sweden. The boiler is used for incineration of solid waste fuels. Two fluidized bed heat exchangers are located in loop seals, connecting the cyclones and the furnace. The heat exchangers are placed in series, with respect to the steam side, and in parallel, with respect to the particle side. The total heat transfer surface area is roughly 44 m2, distributed over 72 tubes. The total effect transferred most often is in the range 2–6 MW. The incoming steam temperature in the first superheater is 380–400 °C, while the exiting steam temperature from the second is around 450°, at 65 bar pressure. The bed temperature in the Fluidized Bed Heat Exchanger (FBHE) is 850–875 °C. The analysis is based on operational data from two time periods (2002–2005 and 2014–2021). The two periods use different heat exchanger designs, following a retrofit in 2005. The aim of the study is to establish the bed-to-tube heat transfer coefficient in an industrial FBHE unit and investigate how it varies over different time periods, for two different bed materials and for two different designs. Also, the experimentally determined heat transfer coefficients are compared with an established heat-transfer correlation, for prediction of heat transfer from bubbling fluidized bed to tubes. Operation with two bed materials were evaluated, namely silica sand and crushed and beneficiated ilmenite. Both materials are classified as Geldart B particles. Air is used as fluidization gas in the FBHE. The analysis show, with a few exceptions, comparably low heat-transfer coefficients from bed to tube of 100–150 W/(m2K). The results were similar for silica sand and ilmenite, but the highest measured heat transfer coefficient was for a period with ilmenite. The heat transfer was lower than expected based on literature data from FBHE units and fluidized bed boilers in general, and much lower than bed-to-tube heat transfer coefficients from lab-scale experiments and empirically derived predictive expressions. The difference could be related to one or more of several factors, such as the effect of very small tube spacing, unknown thermal conductivity of one of the layers in the tube bundle, the effect of lateral particle flow and the effect of fouling due to ash layers forming on the tube surfaces. It is suggested that it should be possible to significantly increase the bed-to-tube heat transfer by increasing the tube pitch, which is expected to improve bed mixing without increasing the risk of corrosion.

Study of effect of geometrical and thermodynamic variables on the performance of condenser in an organic rankine cycle

The study focusses on parametric optimization of condenser used in Organic Rankine Cycle (ORC) to recover waste heat from industrial exhaust. The thermal properties of R245fa are obtained from REFROP-9.0 and the calculation as per the mathematical model has been carried out using MATLAB 2019. Effect on variables is studied on the total heat transfer surface area and cost with channel distance, width of channel, length of the plate, mass flow rate and velocity of refrigerant. Furthermore, effect of mass velocity of refrigerant is studied and the trade-off between the two objective function is presented. The trends observed in the results are presented where there is a trade-off between cost and the total area of heat exchanger. The mass flow rate and mass velocity have significant effects on optimizing design of condenser because a small change results in huge changes in the area of heat exchanger and cost, whereas channel distance, channel width, velocity and mass velocity of R245fa have less effect. The mass flow rate of R245fa has a promising effect in optimizing design of condenser for a minimum value of 3.5 kg/s, the change in the value of the area is 90 m2 and 12,500 euros in cost. Similarly, when the parametric factor of mass velocity is around 120 kg/m2 s, minimal cost is attained and the area observed is around 86 m2 whereas maximum cost of 11,800 euros is seen at mass velocity of 40 kg/m2s.

LNG 냉열을 회수하기 위해 재킷 냉각수를 열원으로 하는 유기랭킨사이클의 열교환기 설계

In this study, the thermal design of the evaporator and condenser in the ORC system using the jacket cooling water as the high temperature heat source and the cooling heat of LNG as the low temperature heat source was carried out. R134a was used as the working fluid for the ORC system. In the case of the evaporator, Shah"s correlation shows the lowest heat transfer coefficient among the two-phase flow heat transfer coefficients in tubes obtained from the literature. It was found that the contribution of nuclear boiling in the low quality region was dominant. Kandlikar"s correlation also showed the contribution of nuclear boiling. As the quality is increased, the contribution of convective evaporation was not significant. Lim"s correlation showed the highest heat transfer coefficient, and the contribution of nuclear boiling in the whole quality region was not as large as that of convective evaporation. Therefore, the total heat transfer surface area of the evaporator was the highest when obtained from Shah"s correlation. Low value was obtained when obtained from the correlation of Lim. In the condenser, the shell-side heat transfer coefficient obtained from Nusselt analysis was the lowest, followed by Kern and Eissenberg. The heat transfer surface area of the condenser showed the highest when obtained from Nusselt analysis.

Three-objective shape optimization and parametric study of a micro-channel heat sink with discrete non-uniform heat flux boundary conditions

A water-cooled multi-die heat sink with parallel rectangular micro-channels was designed to satisfy the operational requirements of a multi-die processor. A shape optimization strategy based on the RSM (response surface method) was used to minimize pressure drop and die maximum case temperatures. The effects of the thermal interface materials and heat spreader between the dies and heat sink were captured by the numerical simulation. The optimization was performed for constant values of coolant flow rate and inlet temperature, as well as the power, location, and surface area of the dies. The influence of channel hydraulic diameter, Reynolds number, thermal entrance length, and total heat transfer surface area on the hydraulic and thermal performance of the heat sink was determined using CFD (computational fluid dynamics) simulations at RSM design points. A sensitivity analysis was performed to evaluate the effect of the design parameters on the response parameters. The optimum designs were achieved by minimizing a weighted objective function defined based on response parameters using JAYA algorithm. The results of weighted sum method were compared with Pareto based three-objective optimization with a NSGA-II (non-dominated sorting GENETIC algorithm). Finally, a parametric study was performed to see the effect of the design parameters on the response parameters.

Optimal Performance Characteristics of Subcritical Simple Irreversible Organic Rankine Cycle

Based on the theory of finite time thermodynamics, a subcritical simple irreversible organic Rankine cycle (SSIORC) model considering heat transfer loss and internal irreversible losses is established in this paper. The total heat transfer surface area is taken as a constraint, and R245fa is adopted as working fluid of the cycle in the performance optimization. The evaporator heat transfer surface area and mass flow rate of the working fluid are optimized to obtain the maximum power output and thermal efficiency of the SSIORC, respectively. In addition, the influences of the internal irreversibilities on the optimal performances are also investigated. The results show that when the evaporator heat transfer surface area is varied, the relationship between power output and thermal efficiency is a loop-shaped curve, and there exist maximum power output and thermal efficiency points, respectively. However, the two maximum points are very close to each other. When the mass flow rate of the working fluid is varied, the relationship between power output and thermal efficiency is a parabolic-like curve. With the decreases of expander and pump irreversible losses, the performances of the irreversible SSORC are close to those of the endoreversible SSORC with the only loss of heat transfer loss.

Investigating the effect of nanoparticle on thermo-economic optimization of fin and tube heat exchanger

In this paper, the effects of Al2O3 nanoparticles suspended in the water-based fluid on the thermo-economic properties of a fin and tube heat exchanger are studied using fast and elitism nondominated sorting genetic algorithm. Nine design parameters are selected as design parameters, and the total annual cost and effectiveness are considered as the two objective functions. First, the effect of nanoparticle on the total annual cost versus effectiveness is obtained at different cold side mass flow rates, and the results are compared with the base fluid. The results show that nanoparticles have a significant influence on the total annual cost and effectiveness in a lower cold side mass flow rates. Next, the heat exchanger volume versus effectiveness for the optimum points is measured at different cold side mass flow rates. It is demonstrated that, adding Al2O3 nanoparticle to the base fluid for the fixed value of effectiveness, decreases the heat exchanger volume, and this reduction is more significant in the lower mass flow rates. The pressure drop and total heat transfer surface area versus effectiveness for the optimum points are also obtained with and without nanoparticle. An increase in the tube side pressure drop is revealed in the nanofluid. In addition, due to the increase in the overall heat transfer coefficient, the lower heat transfer surface area is required for the fixed value of effectiveness. Finally, variations of objective functions versus particle volumetric concentration for five typical optimum points are estimated. It is concluded that an optimal value for the volumetric concentration can be obtained, in which the effectiveness is highest.

Material Selection and Fluid Flow Analysis of Parallel Flow Heat Exchanger

Aims: To select appropriate material and undertake fluid flow analysis of parallel flow heat exchanger. Methodology: Dynamic simulation study was performed to evaluate conditions of heat transfer of water through heat exchanger using ASPEN HYSYS. Cambridge Education Selector (CES) Granta software was used to select the suitable candidate materials for the heat exchanger. The properties of the selected materials and fluid characteristics were implemented in Computational Fluid Dynamics (CFD) to solve and analyze the fluid flow. Results: For inner pipe, low alloy steel having maximum thermal conductivity of 55(W/m°C) and specific heat capacity of 530 (J/kg°C) respectively was selected and used. For the outer pipe, cast iron, gray having thermal conductivity of 72 (W/m°C) and specific heat capacity of 495(J/kg°C) passed the selection criteria and was used.The total heat transfer surface area and hydraulic diameter was determined as 0.1341634m2 and 0.481m respectively while the velocity of flow (V) for hot and cold fluid was established as 0.00014518 m/sec and 0.000196817 m/sec respectively. Conclusion: The material-process decision on a parallel flow heat exchanger design was reduced to trade-off between performance and cost. The data obtained from the experimental record are well matched with computational fluid dynamics simulated values at different mass flow rate.

Performance comparison of the liquid–vapor separation, parallel flow, and serpentine condensers in the organic Rankine cycle

Organic Rankine cycle (ORC)-based power generation is a promising technology for recovering waste energy and effectively utilizing renewable energy with low enthalpy. A condenser is an important unit in ORC. The screening and design optimization of the condenser is significant in achieving high efficiency and low cost. Liquid–vapor separation (LSC) is a newly developed air-cooled fin-tube condenser with low pressure drop, low investment cost, and high compactness. The advantages of LSC have been proven in the refrigeration system. This study theoretically investigates the comprehensive performance of LSC in ORC. A mathematical model of LSC and an ORC that contains LSC is formulated. The effects of some key geometric parameters on the total heat transfer surface area and pressure drop of the condenser are examined. The total cost of investment and operation is selected as the objective function to identify the comprehensive performance of the heat transfer coefficient and presser drop at a constant heat transfer rate. The LSC is also compared with the serpentine condenser (SC) and parallel flow condenser (PFC) in terms of heat transfer coefficients, pressure drop, heat transfer area, and economic cost. The systematic performance of ORC containing different types of condensers is examined and compared in the context of ORC at fixed heat resource parameters and environment parameter. The structural parameter analysis shows that the tube length, tube inner diameter and tube pass arrangement with minimum cost of LSC are 1.5 m, 11 mm and 17-15-10-5-1, respectively. The optimal total cost of LSC is 3.74% and 34.50% lower than that of PFC and SC under a given design condition. The comparison of ORC with different condensers shows that the thermal efficiency (exergy efficiency) of ORC-LSC and ORC-PFC are 0~13.75% (0~11.82%) and 25.25~65.53% (21.83~52.3%) higher than those of ORC-SC, respectively. The sensitivity analysis of heat resource parameters and environment parameter is also performed, and quantitative comparisons of three ORCs are provided.

Determination of Optimum Surface Distribution in Triple-Effect Evaporator

Subjected to fixed total heat transfer surface area, the distribution of surface in multiple-effect evaporator affects the performance of the evaporator. Previous studies have been concerned with optimization of surface distribution with the assumption that no fouling occurs. The present study considers a more realistic case when the last-effect vessel is subjected to fouling. It is shown that, in order to consume the minimum steam exergy to produce a given amount of evaporation, the area of the last-effect vessel must be increased as the rate of fouling increases.

Optimum performance analysis of a combined thermionic-thermoelectric refrigerator with external heat transfer

An irreversible combined thermionic-thermoelectric refrigerator model with external finite rate heat transfer is established. The general expressions for cooling load and coefficient of performance (COP) versus applied voltage are derived. The performance of the irreversible combined refrigerator, in which the heat transfer between the device and the heat reservoir obeys Newton's heat transfer law, is analyzed and optimized by using the combination of finite time thermodynamics and non-equilibrium thermodynamics. The influence of external heat transfer is investigated by comparing the performance of irreversible combined refrigerator with the traditional analysis without heat transfer losses. The performances of the combined refrigerator device with external heat transfer are further compared with those of an independent vacuum thermionic refrigerator with and without considering external heat transfer, respectively. Moreover, for the fixed total heat transfer surface area of four heat exchangers, the allocations of the heat transfer surface area among the four heat exchangers are optimized for maximizing the cooling load and the COP. The effect of total heat transfer surface area on the optimum performance of the irreversible refrigerator is explored by numerical examples. The results obtained herein may provide guidelines for the design and application of practical combined thermionic-thermoelectric refrigeration devices.

Multi-objective optimization design of condenser in an organic Rankine cycle for low grade waste heat recovery using evolutionary algorithm

The optimum design of a condenser is significant in an organic Rankine cycle to achieve higher waste heat utilization efficiency. Based on the mathematical model of a condenser using plate heat exchanger (PHE), some key geometric parameters on the total heat transfer surface area and pressure drop of the condenser are examined. In order to obtain geometric parameters of a plate heat exchanger, a multi-objective optimization of the condenser in organic Rankine cycle is conducted to achieve the optimal geometry design. The total heat transfer surface area and pressure drop are selected as two objective functions to minimize both total heat transfer surface area and pressure drop under the constant heat transfer rate and LMTD conditions. The plate width, plate length and plant distance are selected as the decision variables. Non-dominated sorting generic algorithm-II (NSGA-II) which is an effective multi-objective optimization method is employed to solve this multi-objective optimization design of PHE. The results show that an increase in channel distance or plate width increases the total heat transfer surface area and decreases pressure drop in the condenser. It is noted that the plate length of PHE has a positive effect on the optimization design of PHE. By multi-objective optimization design of the PHE, a Pareto optimal point curve is obtained, which shows that a decrease in total heat transfer surface area of a condenser can increase the pressure drop through the condenser.

Optimum variables selection of thermoelectric generator-driven thermoelectric refrigerator at different source temperature

Based on the finite time thermodynamic model of thermoelectric generator-driven thermoelectric refrigerator with losses of external heat transfer, Joulean heat inside the thermoelectric device and the heat leakage through the thermoelectric couple leg, this paper analysed the effects of generator heat source temperature and refrigerator cooling temperature on the performance of the combined system using the combination of finite time thermodynamics and non-equilibrium thermodynamics. For a fixed total heat transfer surface area of four heat exchangers, the allocations of the heat transfer surface area among the four heat exchangers are optimised for maximising cooling load and the coefficient of performance (COP) at different source temperature of the combined thermoelectric refrigerator device, respectively. For a fixed total number of thermoelectric elements, the ratio of number of thermoelectric elements, i.e. the number of thermoelectric elements of the generator to that of the whole device is also optimised for maximising cooling load and the COP at different source temperature of the combined device, respectively. The change features of four design variables, i.e. the ratio of number of thermoelectric elements, the total heat transfer surface area ratio, the generator heat transfer surface area ratio and the refrigerator heat transfer surface area ratio are obtained, respectively. Moreover, optimum working electrical currents for maximum cooling load and COP at different total number of thermoelectric elements and different total heat transfer area are obtained, respectively.

Performance Analysis and Optimization of a Single-Barrier Solid-State Thermionic Refrigerator With External Heat Transfer

A model of a single-barrier solid-state thermionic refrigerator with external heat transfer is established in this paper. The performance of the refrigerator is analyzed and optimized by using the combination of finite-time thermodynamics and nonequilibrium thermodynamics. The general expressions for cooling load and coefficient of performance (COP) of the refrigerator are derived. The optimum regions of cooling load and COP are obtained and the effects of the heat reservoir temperature and thermal conductance of the barrier material on the performance of the refrigerator are analyzed by detailed numerical examples. The results obtained are compared with those obtained by using traditional analysis without considering external heat transfer. For the fixed total heat transfer surface area of two heat exchangers, the ratios of the heat transfer surface area of the hot-side heat exchanger to the total heat transfer surface area of the heat exchangers are optimized for maximizing the cooling load and COP of the refrigerator, respectively. The effects of the total heat transfer surface area and the applied voltage on the optimum performance of the refrigerator are analyzed. The results obtained herein may provide some theoretical guidelines for the design and application of practical solid-state thermionic refrigerators.

Effect of heat transfer on the performance of thermoelectric generator-driven thermoelectric refrigerator system

A model of thermoelectric generator-driven thermoelectric refrigerator with external heat transfer is proposed. The performance of the combined thermoelectric refrigerator device obeying Newton’s heat transfer law is analyzed using the combination of finite time thermodynamics and non-equilibrium thermodynamics. Two analytical formulae for cooling load vs. working electrical current, and the coefficient of performance (COP) vs. working electrical current, are derived. For a fixed total heat transfer surface area of four heat exchangers, the allocations of the heat transfer surface area among the four heat exchangers are optimized for maximizing the cooling load and the coefficient of performance (COP) of the combined thermoelectric refrigerator device. For a fixed total number of thermoelectric elements, the ratio of number of thermoelectric elements of the generator to the total number of thermoelectric elements is also optimized for maximizing both the cooling load and the COP of the combined thermoelectric refrigerator device. The influences of thermoelectric element allocation and heat transfer area allocation are analyzed by detailed numerical examples. Optimum working electrical current for maximum cooling load and COP at different total number of thermoelectric elements and different total heat transfer area are obtained, respectively.

Analytical modeling of water condensation in condensing heat exchanger

An analytical model of heat and mass transfer processes in a flue gas condensing heat exchanger system was developed to predict the heat transferred from flue gas to cooling water and the condensation rate of water vapor in the flue gas. Flue gas exit temperature, cooling water outlet temperature, water vapor mole fraction, and condensation rate of water vapor were computed using the analytic model. A pilot-scale heat exchanger was used to validate the analytical model. The experimental results show a very good agreement with analytical model results. The performance of the heat exchanger system was evaluated as functions of the ratio of the mass flow rate of cooling water to the mass flow rate of inlet flue gas, the inlet cooling water temperature and the ratio of flue gas flow rate to total heat transfer surface area.

High Performance Recuperator With Oblique Wavy Walls

A series of numerical simulation of the flow and heat transfer in modeled counterflow heat exchangers with oblique wavy walls has been made toward optimal shape design of recuperators. The effects of oblique angles and amplitudes of the wavy walls are systematically evaluated, and the heat transfer and pressure loss characteristics are investigated. It is found that counter-rotating streamwise vortices are induced by the wavy walls, and the flow field has been drastically modified due to the intense secondary flow. By using the optimum set of oblique angle and wave amplitude, significant heat transfer enhancement has been achieved at the cost of relatively small pressure loss, and the j∕f factor becomes much larger than that of straight square duct or conventional compact recuperators. When thermal coupling of hot and cold fluid passages is considered, the heat transfer is found to be strongly dependent on the arrangement of counterflow passages. The total heat transfer surface area required for a given pumping power and heat transfer rate can be reduced by more than 60% if compared to the straight square duct.

Optimum allocation of heat transfer surface area for heating load and COP optimisation of a thermoelectric heat pump

SYNOPSIS The combination of finite time thermodynamics and non-equilibrium thermodynamics is applied to analyse and optimise the performance of a thermoelectric heat pump, which is composed of multi-elements. For a fixed total heat transfer surface area of two heat exchangers, the ratio of heat transfer surface area of the high temperature side heat exchanger to the total heat transfer surface area of the heat exchangers is optimized for maximising the heating load and the coefficient of performance (COP) of a thermoelectric heat pump. The effects of various parameters on the optimum performance are analysed. The results may provide guidelines for the analysis and optimisation of practical thermoelectric heat pumps.

Heat transfer surface area optimization for a thermoelectric generator

SYNOPSIS The combination of finite time thermodynamics and non-equilibrium thermodynamics is applied to analyse and optimise the performance of a thermoelectric generator, which is composed of multi-elements. For the fixed total heat transfer surface area of two heat exchangers, the ratio of heat transfer surface area of the high temperature side heat exchanger to the total heat transfer surface area of the heat exchangers is optimised for maximising the power output and the thermal efficiency of the thermoelectric generator. The effects of various parameters on the optimum performance are analysed. The results may provide guidelines for the analysis and optimisation of practical thermoelectric generators.