Ratio Of Heat Transfer Area Research Articles

Parametric Optimization of Organic Rankine Cycle by Genetic Algorithm

Organic Rankine Cycle (ORC) is widely used in the field of low temperature waste heat recovery, including solar, biomass and geothermal energy, among others. Based on the thermodynamic model of ORC system built up in Matlab, this study employ Genetic Algorithm (GA) on ORC system for parametric optimization and select a ratio of heat transfer area to total net power output as the performance evaluation criterion to predict the economy of system. R11, R113, R123 and isopentane are choosed as the working medium. The results show that the ORC system with isopentane has the minimum objective function value of 0.429m2/kw. The corresponding condensing temperature and degree of supercooling are generally located at lower boundary over their parametric design ranges, and the corresponding pinch point temperature difference are located at upper boundary. For different working fluids, there exist an optimum evaporating temperature and degree of superheat.

Optimization of plate-fin heat exchangers by minimizing specific entropy generation rate

In the present study, an optimization model is developed for plate-fin heat exchangers (PFHEs) based on the entropy generation minimization method. In the modeling, the entropy generation rate per unit partition plate area, i.e., specific entropy generation rate is proposed as an optimization objective function and the total heat transfer area of a PFHE is considered as a constraint. The model takes into account the irreversibility due to both irreversible heat transfer and pressure drop. The analysis of optimal allocation of total heat transfer area between the hot- and cold-side for a PFHE is conducted with a case study. The results show that optimal geometric parameters of a PFHE such as the fin spacing and fin height on both the hot- and cold-sides can be achieved when the finite total heat transfer area is optimally allocated with minimizing the specific entropy generation rate. In addition, the effects of total heat transfer area, heat transfer rate, as well as the ratio of mass flow rate between the cold- and hot-side fluid on the specific entropy generation rate and optimal allocation ratio of total heat transfer area are also analyzed.

A thermodynamic analysis of waste heat recovery from reciprocating engine power plants by means of Organic Rankine Cycles

Organic Rankine Cycle (ORC) is a Rankine cycle using organic fluid as the working fluid instead of water and steam. The ORC process is a feasible choice in waste heat recovery applications producing electricity from relatively low-temperature waste heat sources or in applications having a rather low power output. Utilizing waste heat from a large high-efficiency reciprocating engine power plant with ORC processes is studied by means of computations. In addition to exhaust gas heat recovery, this study represents and discusses an idea of directly replacing the charge air cooler (CAC) of a large turbocharged engine with an ORC evaporator to utilize the charge air heat in additional power production. A thermodynamic analysis for ORCs was carried out with working fluids toluene, n-pentane, R245fa and cyclohexane. The effect of different ORC process parameters on the process performance are presented and analyzed in order to investigate the heat recovery potential from the exhaust gas and charge air. A simplified feasibility consideration is included by comparing the ratio of the theoretical heat transfer areas needed and the obtained power output from ORC processes. The greatest potential is related to the exhaust gas heat recovery, but in addition also the lower temperature waste heat streams could be utilized to boost the electrical power of the engine power plant. A case study for a large-scale gas-fired engine was carried out showing that the maximum power increase of 11.4% was obtained from the exhaust gas and 2.4% from the charge air heat.

An optimal design method for a heat exchanger for a dual-stage circulating high-temperature heat pump system

This paper presents an optimal design method for heat exchangers in a dual-stage circulating high-temperature heat pump (HTHP) system. The paper first presents a series of steady-state models of components and a simulation platform of whole system; the output of the simulated system is verified by the operational data of a real heat pump plant. The method is investigated and implemented by the simulation platform based on these models, and it consists of four steps: 1) the selection of an optimal condenser and evaporator (the C-E plant), 2) the optimization of the total heat transfer area of the C-E plant, 3) the optimization of the condenser-to-evaporator heat transfer area ratio (AR) and 4) a matching scheme for the AR and heat transfer area of the regenerator. The method adopts the coefficient of performance (COP) as the optimization target and employs the heating capacity, water velocities and water-side pressure drops and the compressor discharge temperature as constraints. The case studies demonstrate that the presented method is effective at solving the problem of mismatches among single components. The optimal design scheme provides the same heating capacity with a 13% decrease in the total area of the exchangers.

Fluids and parameters optimization for the organic Rankine cycles (ORCs) used in exhaust heat recovery of Internal Combustion Engine (ICE)

An Organic Rankine Cycle (ORC) system used in the Internal Combustion Engine (ICE) exhaust heat recovery was proposed and techno-economically analyzed based on various working fluids. It is significant to recover ICE exhaust heat (about one third of energy generated from the fuel) by ORC system. In this paper, the suitable working fluids have been screened and recommended for the ORC system, among 20 fluids (boiling point temperature range from −51.60 to 32.05 °C) analyzed on the rated condition of one popular commercial diesel generator set. The cycle parameters, including the thermal efficiency (ηth), the expansion ratio (v2/v1), the net power output per unit mass flow rate of hot exhaust (Pnet), the ratio of total heat transfer area to net power output (A/Wnet), and electricity production cost (Epc), have also been analyzed and optimized. Results show that R141b, R123 and R245fa present the highest ηth and Pnet values ranging from 16.60% to 13.30% (ηth value), and from 60 to 49 kJ/kg (Pnet value). Meanwhile, the three fluids also express the lowest Epc values ranging from 0.30 to 0.35$/kWh, and lowest A/Wnet values ranging from 0.436 to 0.516 m2/kW. The optimum evaporating pressures for R141b, R123, R245fa are ranging from 2.8 MPa to 3.6 MPa.Research Subject: Waste heat recovery technology in internal combustion engine; The key technology on the improvement of ICE efficiency.

Parametric optimization and performance comparison of organic Rankine cycle with simulated annealing algorithm

Taking the ratio of heat transfer area to net power and heat recovery efficiency into account, a multi-objective mathematical model was developed for organic Rankine cycle (ORC). Working fluids considered were R123, R134a, R141b, R227ea and R245fa. Under the given conditions, the parameters including evaporating and condensing pressures, working fluid and cooling water velocities were optimized by simulated annealing algorithm. The results show that the optimal evaporating pressure increases with the heat source temperature increasing. Compared with other working fluids, R123 is the best choice for the temperature range of 100–180 °C and R141b shows better performance when the temperature is higher than 180 °C. Economic characteristic of system decreases rapidly with the decrease of heat source temperature. ORC system is uneconomical for the heat source temperature lower than 100 °C.

Fluids and parameters optimization for a novel cogeneration system driven by low-temperature geothermal sources

A novel cogeneration system was proposed and techno-economically investigated, consisting of a low-temperature geothermally-powered organic Rankine cycle (ORC) subsystem, an intermediate heat exchanger subsystem and a heat pump subsystem. The main purpose is to identify suitable working fluids (among 27 fluids with boiling point temperature ranging from −47.69 to 47.59 °C) and optimized cycle parameters for the ORC-based power generation subsystem. The screening criteria include net power output per unit mass flow rate of hot source ( P net), the ratio of total heat transfer area to net power output ( A/ W net), and electricity production cost (epc). Results show that there exists optimum evaporating temperatures maximizing the P net value and minimizing the A/ P net and epc values. The optimum temperatures vary with different screening criteria and fluids. Optimized fluids based on each screening criteria are not the same. E170, R600 and R141b show the lowest A/ W net and epc values with averagely 3.78% lower P net value than R236ea which presents the largest P net value.

Full-Field Flow Measurements and Heat Transfer of a Compact Jet Impingement Array With Local Extraction of Spent Fluid

Jet impingement cooling is widely used due to the very high heat transfer coefficients that are attainable. Both single and multiple jet systems can be used, however, multiple jet systems offer higher and more uniform heat transfer. A staggered array of 8.46 mm diameter impingement jets with jet-to-jet spacing of 2.34 D was examined where the spent fluid is extracted through one of six 7.36 mm diameter extraction holes regularly located around each jet. The array had an extraction area ratio (Ae/Ajet) of 2.23 locally and was tested with a jet-to-target spacing (H/D) of 1.18 jet diameters. Magnetic resonance velocimetry was used to both quantify and visualize the three dimensional flow field inside the cooling cavity at jet Reynolds numbers of 2600 and 5300. The spatially averaged velocity measurements showed a smooth transition is possible from the impingement jet to the extraction hole without the presence of large vortical structures. Mean Nusselt number measurements were made over a jet Reynolds number range of 2000–10,000. Nusselt numbers near 75 were measured at the highest Reynolds number with an estimated uncertainty of 7%. Large mass flow rate per unit heat transfer area ratios were required because of the small jet-to-jet spacing.

Experimental evaluation of commercial heat exchangers for use as PCM thermal storage systems

Phase change materials (PCM) possess a great capacity of accumulation of energy in their temperature of fusion thanks to the latent heat. These materials are used in applications where it is necessary to store energy due to the temporary phase shift between the offer and demand of thermal energy. Thus, possible applications are the solar systems as well as the recovery of residual heat for its posterior use in other processes. In spite of this great potential, the practical feasibility of latent heat storage with PCM is still limited, mainly due to a rather low thermal conductivity. This low conductivity implies small heat transfer coefficients and, consequently, thermal cycles are slow and not suitable for most of the potential applications. This work investigates experimentally the heat transfer process during melting (charge) and solidification (discharge) of five small heat exchangers working as latent heat thermal storage systems. Commercial paraffin RT35 is used as PCM filling one side of the heat exchanger and water circulates through the other side as heat transfer fluid. Average thermal power values are evaluated for various operating conditions and compared among the heat exchangers studied. When the comparison is done for average power per unit area and per average temperature gradient, results show that the double pipe heat exchanger with the PCM embedded in a graphite matrix (DPHX-PCM matrix) is the one with higher values, in the range of 700–800 W/m 2-K, which are one order of magnitude higher than the ones presented by the second best. On the other hand, the compact heat exchanger (CompHX-PCM) is by large the one with the highest average thermal power (above 1 kW), as it has the highest ratio of heat transfer area to external volume.

마이크로핀관에서의 냉매 R-22, R-407C의 응축전열특성에 관한 연구

Experimental results for forced convection condensation of Refrigerant-22 and ternary Refrigerant-407C(HFC-32/125/134a 23/25/52 wt%) which is being considered as a substitute R-22 inside a horizontal micro-fin tube are presented. The test section was horizontal double-tube counterflow condenser with a length 4,000 mm micro-fin tube, having 8.53 mm ID, 0.2 mm fin height and 60 fins. The range of parameters of mass velocity were varied from 102.1 to 301.0 kg/(m2.s) and inlet quality 1.0. At the given experimental conditions. the average heat transfer coefficients for R-407C were lower than that for R-22 at a micro-fin tube. Over the mass velocity range tested. the PF(penalty factor) for R-22, R-407C were lower than the increasing ratio of heat transfer area by fins, and the EF(enhancement factor) for R-22, R-407C were higher than the increasing ratio of heat transfer area by fins.

Effect of posture on the heat transfer areas of the human body

The purpose of this paper is to measure the heat transfer areas of the human body and to examine the effect of posture on these values, which is necessary data for calculating heat exchange between the human body and its environment. The total surface area of a subject's body was measured directly. Then, the convective heat transfer area, radiative heat transfer area and conductive heat transfer area were measured for the same subject in 9 postures: standing, chair sitting, seiza sitting, cross-legged sitting, sideways sitting, both-knees-erect sitting, legs-out sitting, lateral position and supine. The ratios of the radiative heat transfer area, convective heat transfer area ratio and conductive heat transfer area to body surface area were as follows: Standing, 0.942, 0.773, 0.013; chair sitting, 0.910, 0.732, 0.008; seiza sitting, 0.853, 0.621, 0.013; cross-legged sitting, 0.843, 0.606, 0.029; sideways sitting, 0.877, 0.634, 0.030; both-knees-erect sitting, 0.865, 0.609, 0.023; legs-out sitting, 0.878, 0.686, 0.038; lateral position, 0.879, 0.712, 0.039; and supine, 0.811, 0.708, 0.100. Posture was shown to have a noticeable effect on the heat transfer areas of the human body.

Performance investigation of thermoelectrics with an external spacer inserted in serious

This study consists of two parts: mathematical modeling for the start-up period (transient system) and an optimum design analysis of a thermoelectric cooler (TEC). In the mathematical modeling for the transient system, a dimensionless time to reach the steady state is determined, which can be practically adopted to reduce the start-up period during the operation of a thermoelectric module (TEM). The objectives of the optimum design analysis are to investigate the effect of key parameters characterizing the TEC on the coefficient of performance ( COP) and to determine the optimal conditions for the TEC design. It is found that the COP increases up to 44% with an increase of the ratio of heat transfer area of the insulator to that of the TEM, γ. For micro-TEC, the COP is significantly affected by the TEM depth and spacer. Especially in the case of TEM depth of 100 μm, the COP is enhanced up to 110% at an optimal spacer depth ratio ( ε − 1 ) of 10. The results show that the effect of figure of merit Zm ( = S 2 / R K TE ) on the COP appears differently according to the combination of thermal conductance K TE , electric resistance R and Seebeck constant S of the TEM. It reveals that when a new thermoelectric material is developed, the optimal combination among K TE , R , and S as well as Zm should be considered simultaneously.

Effects of combined heat transfer on the thermo-economic performance of irreversible solar-driven heat engines

A thermo-economic performance analysis and optimization has been carried out for an irrversible solar-driven heat engine with losses due to heat transfer across finite temperature differences, heat leak and internal irreversibilities. In the considered heat engine model, heat transfer from the hot reservoir is assumed to be simultaneous radiation and convection mode and the heat transfer to the cold reservoir is assumed to be convection mode. The effects of the technical and economical parameters on the thermo-economic performance have been investigated in order to see the collective effects of the radiation and convection modes of heat transfer. Also the optimal performance parameters of the heat engine, such as the thermal efficiency, temperatures of the working fluid and the ratio of heat transfer areas have been discussed in detail.

Overall conductance and heat transfer area minimization of refrigerators and heat pumps with finite heat reservoirs

In the present study, the overall conductance and the overall heat transfer area per unit capacity of refrigeration and heat pump systems have been minimized analytically considering both internal and external irreversibilities with variable temperature (finite capacity) heat reservoirs. Hot and cold side refrigerant temperatures, conductance and heat transfer area ratios have been optimized to attain this goal. The results have been verified with the more elaborate numerical optimization results obtained for ammonia based vapour compression refrigeration and heat pump systems working with variable temperature reservoirs. It is observed that the analytical results for optimum refrigerant temperatures, minimum overall conductance and heat transfer area deviate marginally from the numerically optimized results (within 1%), if one assumes a constant heat rejection temperature. The deviation of minimum overall conductance and heat transfer area is more (about 20%), if one considers both the desuperheating and condensation regions separately. However, in the absence of complex and elaborate numerical models, the simple analytical results obtained here can be used as reasonably accurate preliminary guidelines for optimization of refrigeration and heat pump systems.

Important optimum criteria on the performance parameters of an irreversible Brayton heat engine

SYNOPSIS An irreversible cycle model of a Brayton heat engine is presented in this paper. The internal irreversibility is due to nonisentropic processes in the compressor and turbine while the external irreversibility is due to finite temperature differences between the external heat reservoirs and the heat engine. The power output is maximised with respect to the working fluid temperatures and the pressure ratio of two isobaric processes. It is found that there are optimal values of the pressure ratio and cycle temperatures at which the power output attains its maximum for a typical set of operating parameters. Some optimum criteria for the important parameters such as power output, thermal efficiency, pressure ratio, temperatures of the working fluid in the two isobaric processes, and heat transfer area ratios, are obtained. The important problems concerning the optimal design and operation of Brayton heat engines are discussed in detail.

THE OPTIMUM ALLOCATION OF HEAT TRANSFER EQUIPMENT FOR AN IRREVERSIBLE COMBINED HEAT ENGINE WITH ECOLOGICAL CRITERIA

Finite-time thermodynamics with an ecological criterion is used to optimize the performance of an irreversible combined Carnot heat-engine. The irreversiblities come from two sources: (1) finite heat conductance for the three heat exchangers; (2) irreversibility inside the heat engine. The ecological function is defined as the power output minus the loss power or the product of the environmental temperature and the entropy rate. The ecological function is optimized with respect to two dimensionless cycle temperatures and thermal efficiency. The maximum ecological function and its corresponding power output and thermal efficiency are presented. It is shown that the ecological function is an important criterion for the design of an irreversible combined Carnot heat engine, considering not only the power output but also the thermal efficiency. When the three heat exchangers have equal heat transfer coefficients, the optimum heat-transfer area ratio for the middle heat exchanger is always equal to 1/3, independent of the irreversibility parameters for the top and bottom cycles and the heat reservoir temperature ratio. Furthermore, the optimum heat-transfer area ratio for the hotend heat exchanger decreases while that for the cold-end heat exchanger increases as the irreversibility parameters of the top and bottom cycles are increased.

Weighting Coefficients for Calculating Mean Skin Temperature when Considering Convective Heat Tranfer Areas

When accurately quantifying convective heat exchange between the human body and the surrounding environment, mean skin temperature is determined by measuring the skin temperature of areas of the body surface exposed to airflow (hereinafter referred to as the convective heat transfer area) and then calculating the ratio of the convective heat transfer area with the uniform skin temperature to the total convective heat transfer area. Thus we must consider convective heat transfer areas to determine mean skin temperature. In this study, the convective heat transfer area of the human body in two basic postures (standing on the floor and sitting on a chair) was measured by dividing the body surface into smaller sections. We then proposed weighting coefficients for calculating mean skin temperature taking into account convective heat transfer areas, and conducted experiments to clarify how convective heat transfer areas affected calculations of mean skin temperature. The results showed that, for both standing and sitting positions, due to the areas of contact between two body surfaces and between the body and the floor, the convective heat transfer area for the hand and foot regions was markedly small. We then calculated weighting coefficients for calculating mean skin temperature that take into account convective heat transfer areas for standing and sitting positions. The tendency was that the smaller the ratio of convective heat transfer area to the total body surface area, the greater the difference in mean skin temperature using the Hardy-DuBois technique. The results of the present study also clarify that by taking into account convective heat transfer areas, the skin temperature of the extremities can be reflected in mean skin temperature for both standing and sitting positions.

The performance of a new gas to gas heat exchanger with strip fin

A compact gas to gas heat exchanger needs large heat transfer areas on both fluid sides. This can be realised by adding secondary surfaces. The secondary surfaces are plate fin, strip fin, and louvered fin, etc. The fins extend the heat transfer surfaces and promote turbulence. This paper presents a gas to gas heat exchanger with strip fins. The heat exchanger design and construction are based on a method to seal rectangular strip fins in slots in opposite walls of a rectangular pipe. Fins are fixed and sealed to the walls simultaneously by high temperature brazing of glass mixed with metals in a furnace. The additional advantage of glass is that it forms a coating on the heat transfer surface to protect the surface from corrosion. A number of measurements were carried out to test the performance of this heat exchanger. Not surprisingly, the measurement results indicate that heat transfer coefficient and pressure drop increase with the ratio of heat transfer area to volume (fin density). Colburn factor and friction factor versus Reynolds number are presented.

Bi<SUB>2</SUB>Te<SUB>3</SUB>系モジュールを用いた車載用排気熱発電装置

A thermoelectric generator has been made consisting of an inner shell as a exhaust pipe, an outer shell as a water jacket and 16 Bi2Te3 modules working at low temperature established between the inner shell and the outer shell. Heat-exchanging fins were formed inside the inner shell with heat transfer area ratios of 0.92, 1.21, 1.65, and 1.99 along the exhaust gas flow, in order to reduce the temperature distribution at the hot side of the modules without over overheating of the module. When the exhaust gas was introduced to the inner shell of the generator under conditions corresponding to a 2-3 litre gasoline engine vehicle running at 60 km/h up a 3-5% hill, the electric power generated by the generator was 193 W. The heat change conversion efficiency of the generator was estimated at to be 37% of the primary exhaust gas energy flux. The generated power was 2.9% of the heat flux through the generator.

98/00669 Influence of internal heat leak on the performance of refrigerators

In the present study, an optimization model is developed for plate-fin heat exchangers (PFHEs) based on the entropy generation minimization method. In the modeling, the entropy generation rate per unit partition plate area, i.e., specific entropy generation rate is proposed as an optimization objective function and the total heat transfer area of a PFHE is considered as a constraint. The model takes into account the irreversibility due to both irreversible heat transfer and pressure drop. The analysis of optimal allocation of total heat transfer area between the hot- and cold-side for a PFHE is conducted with a case study. The results show that optimal geometric parameters of a PFHE such as the fin spacing and fin height on both the hot- and cold-sides can be achieved when the finite total heat transfer area is optimally allocated with minimizing the specific entropy generation rate. In addition, the effects of total heat transfer area, heat transfer rate, as well as the ratio of mass flow rate between the cold- and hot-side fluid on the specific entropy generation rate and optimal allocation ratio of total heat transfer area are also analyzed.