Oscillating flow behavior of a natural circulation loop using minichannels at atmospheric pressure

Natural circulation loops at the macroscale have been widely applied in the passive cooling of nuclear power plant. However, little has been done on the miniaturized natural circulation loop for electronic cooling. The present study is to develop a miniaturized natural circulation loop consisting of an evaporator, a condenser, a riser (vapor line) and a downcomer (liquid line). Heat is dissipated from the heated chip to the evaporator, and transferred to the condenser by the air natural convection. The working fluid is selected as methanol. It is demonstrated that the system can dissipate the heating power up to 80 W with the temperatures of a simulated heated chip less than 73 °C. With the heating power varying from 10 to 80 W, the loop operates from the oscillating liquid flow to the periodic liquid/two-phase alternate flow. The thermal oscillatings of the simulated heating chip are always random. However, the inlet/outlet fluid temperatures and pressures display periodic oscillating behavior. A single full cycle is identified by the parameter traces and the simple flow visualizations by the naked eye to have three stages: liquid flow stage, sensible heat receiving stage, boiling two-phase discharging stage. These have clear switch points. The oscillating time period can be as long as 57 s at the heating power of 30 W, and is sharply decreased with increasing heating power. It is also shown that the mean wall temperatures only slightly increase with the increasing heating power, providing the better performance of the present natural circulation loop using minichannels at atmospheric pressure.     

U-Tube Assembly Heat Exchanger Performance Analysis Using Cyclic Iteration

This article outlines a technique to predict the performance of the convection passes of pulverized coal boilers, where the heat exchangers are typically comprised of assemblies of U-tube elements. Therein a heat exchanger comprised of n U-tubes per assembly is modeled as 2n rows in the product gas flow direction, and the portions of the straight tubes making up each assembly are considered to be distinct passes in crossflow. It is assumed that in each pass the tube-side fluid is unmixed and the gas-side fluid is mixed, and likewise the gas-side fluid is mixed between passes. The model employs a straightforward approach to predict the performance of a single pass as a building block to predict the performance of the overall assembly.     

一种多用途热泵的分析

通过对现有热泵循环过程的全面分析,提出了一种新型的多用途热泵.该热泵有蒸汽释放出气态显热的高温换热器、工质释放出液化潜热的中温换热器(冷凝器)和工质释放出液态显热的低温换热器.计算表明,与采用相同循环工质的现有热泵相比,该新型热泵具有更高的制热系数.它可以应用于家庭、学校以及企事业等单位的采暖/空调、沐浴以及开水供应等.     

Finite time thermodynamic analysis of an irreversible regenerative closed cycle brayton heat engine

This communication presents the parametric study of an irreversible regenerative Brayton cycle with nonisentropic compression and expansion processes for finite heat capacitance rates of external reservoirs. The power output of the cycle is maximized with respect to the working fluid temperatures and the expressions for maximum power output and the corresponding thermal efficiency are obtained. The effect of the effectiveness of the various heat exchangers and the efficiencies of the turbine and compressor, the reservoir temperature ratio and the heat capacitance rate of heating and cooling fluids and the cycle working fluid on the power output and the corresponding thermal efficiency has been studied. It is seen the effect of cold side effectiveness is more pronounced for the power output while the effect of regenerative effectiveness is more pronounced for the thermal efficiency. It is found that the effect of turbine efficiency is more than the compressor efficiency on the performance of these cycles. It is also found that the effect of sink-side heat capacitance rate is more pronounced than the heat capacitance rate on the source side and the heat capacitance rate of the working fluid.     

EXERGETIC EVALUATION OF GAS TURBINE COGENERATION SYSTEMS FOR DISTRICT HEATING AND COOLING

A cogeneration system (CGS) generating both power and heat for district heating and cooling is required to be able to cope efficiently with its heat demand change. In this paper, two types of gas turbine CGSs were investigated: (1) a CGS using a dual fluid cycle; and (2) a CGS using a combined cycle. Exergy flows at various points of each CGS have been evaluated when its heat demand is changed. The following have been shown through simulation studies: (a) the higher the heat supply ratio, the higher the exergetic efficiency of the dual fluid cycle CGS; (b) the lower the heat supply ratio, the higher the exergetic efficiency of the combined cycle CGS; and (c) the highest exergetic efficiency of the dual fluid cycle CGS at the maximum heat supply operation is higher than that of the combined cycle CGS; and the exergetic efficiency of the combined cycle CGS at the minimum heat supply operation is higher than that of the dual fluid cycle CGS. A simple criterion has also been derived for determining which type of CGS has higher average exergetic efficiency for a specific district when its heat demand characteristics are known. © 1997 by John Wiley & Sons, Ltd.     

Integration of Kalina cycle in a combined heat and power plant,a case study

This paper presents the integration of the Kalina cycle process in a combined heat and power plant for improvement of efficiency. In combined heat and power plants, the heat of flue gases is often available at low temperatures. This low-grade waste heat cannot be used for steam production and therefore power generation by a conventional steam cycle. Moreover, the steam supply for the purpose of heating is mostly exhausted, and therefore the waste heat at a low-grade temperature is not usable for heating. If other measures to increase the efficiency of a power plant process, like feed-water heating or combustion air heating, have been exhausted, alternative ways to generate electricity like the Kalina cycle process offer an interesting option. This process maximizes the generated electricity with recovery of heat and without demand of additional fuels by integration in existing plants. The calculations show that the net efficiency of an integrated Kalina plant is between 12.3% and 17.1% depending on the cooling water temperature and the ammonia content in the basic solution. The gross electricity power is between 320 and 440 kW for 2.3 MW of heat input to the process. The gross efficiency is between 13.5% and 18.8%.     

ANALYSIS OF A CONCEPT FOR INCREASING THE EFFICIENCY OF A BRAYTON CYCLE VIA ISOTHERMAL HEAT ADDITION

A thermodynamic study indicates that the hypothetical modification of gas-turbine engines to include two heat additions rather than one may result in significant efficiency improvements of over 4% compared with conventional engines. Specifically, the usual constant pressure heat addition would be constrained to a given temperature and then further heat addition carried out in a manner approaching an isothermal process. Owing to the limited peak combustion temperature of the overall heat addition process, the emissions of NO_x may be reduced by as much as 50%, thus offering an environmental benefit as well as an efficiency advantage. This paper details the analysis of a proposed combustion chamber in which an isothermal heat addition is approximated. The combustion chamber would consist of a converging duct featuring discrete combustion sites positioned along the streamwise direction. A numerical analysis developed to assess the deviation from isothermal flow in the combustion chamber shows that a reasonable approximation of such a heat addition may be possible with two or more combustion sites. Moreover, a simplified treatment of the combustion process implies that flame stabilization at these sites is feasible. © 1997 by John Wiley & Sons, Ltd.     

Solar-assisted absorption heat pumps feasibility

An absorption system can be used for space cooling as well as for space heating. This dual purpose may be achieved by using the system as heat pump in wintertime. Absorption heat pump heating may be an interesting alternative, particularly for countries where there is a shortage of electric power.When an absorption unit is used as heat pump, its mode of operation is not modified: the internal temperatures of the cycle are only raised. Commercially available LiBr units were tested as heat pumps. COP and heating capacity were considered as a function of cold source temperature for different temperatures of the useful heat. The COP arrived at 1.7, which must be considered a high value for a thermally driven heat pump.Simulations were carried out in order to compare the performance of “conventional” solar, solar assisted heat pump and the combined series system under two different climate conditions. The series system showed performance 25–75 per cent better than “conventional” solar alone.     

Comparison of heating and natural ventilation in a solar house induced by two roof solar collectors

In this paper, two kinds of roof solar collectors (RSCs), namely, the single pass RSC, and the double pass RSC are analyzed and compared. The double pass roof solar collector, which is configured by integrating a double pass solar air collector with the building roof, can be operated more efficiently for space heating in winter, and for natural ventilation in other seasons. To evaluate the effects of two RSCs for both space heating and natural ventilation, a single traditional Chinese style house, on which the two RSCs will be mounted respectively, is developed. Through comparison, it is found that the instantaneous efficiency of solar heat collecting for the double pass RSC is higher than that of the single pass one by 10% on average, and natural ventilation air mass flow rate contributed by natural ventilation for the double pass RSC can be improved to a great extent for most cases, indicating that double pass RSC is superior to the single pass one from the points of view of both space heating and natural ventilation. The double pass RSC is therefore more potential for improving indoor thermal environment and energy saving of buildings.     

不可逆吸收式热泵循环的基本优化关系及性能分析

在恒温热源内可逆四热源吸收式热泵循环的基础上，建立了线性(牛顿)传热定律下考虑泵热空间到环境热源的热漏、工质的内部耗散以及工质与外部热源间的热阻损失的不可逆吸收式热泵循环模型。导出了总换热面积一定的条件下循环的泵热率和泵热系数的基本优化关系、最大泵热率和相应的泵热系数、最大泵热系数和相应的泵热率、以及循环中最佳工质工作温度和最佳换热面积分配关系；并通过数值算例分析了循环参数对循环最优性能的影响规律。     

Analyses of single and double exposure solar air heaters

This paper presents an investigation of the performance of single and double exposure solar air heaters. A conventional solar air heater consist of a flat passage between two metallic plates through which heating fluid (air) is made to pass. the conduction loss along the lengths of the plates in the direction of the air flow and the radiation loss of heat from the absorbing plate to the bottom plate have been incorporated in the analyses. the analyses consist of the exact solutions of the heat balance equations for the absorbing plate, bottom plate and the air stream. Analytical expressions for the plate and the air stream temperatures as a function of distance along the direction of air flow and some other parameters have been derived. It is found that the heat conduction effects are negligible in both the air heaters and the reradiation of heat from the absorbing plate to the bottom plate is also insignificant.     

Performance characteristics of a two-stage irreversible combined refrigeration system at maximum coefficient of performance

A general cycle model of a two-stage combined refrigeration system is established and used for analizing the influence of multi-irreversibilities, such as finite rate heat transfer, heat leak between the heat reservoirs and internal dissipation of the working fluid, on the performance of the refrigeration system. The coefficient of performance is taken as an objective function for optimization. The maximum coefficient of performance is calculated, and other corresponding performance parameters, such as the temperatures of the working fluid in the isothermal processes, the optimal distribution of the heat transfer areas and the power input of the refrigeration system, are determined. The results obtained here are more general than those obtained from a two-stage endoreversible combined refrigeration system and can guide the optimal design and operation of real combined refrigerator systems.     

Interaction of fluid dynamics phenomena and generator efficiency in two-phase liquid-metal gas magnetohydrodynamic power generators

The two-phase-flow (liquid-metal and gas) magnetohydrodynamic (LMMHD) generator and its associated thermodynamic cycle offer a promising alternative to conventional means of electrical power generation. Several advantages can be gained from its application. The LMMHD cycle can be applied to a very wide range of operating temperatures (heat sources) either alone or as a topping or bottoming part of a binary cycle. The potential is high for extreme design simplicity and reliability, because the need for high-speed rotating machinery, as used in turbine power plants, is eliminated. The electrical conductivities, proportional to the power densities, are higher by three or four orders of magnitude than in plasma MHD generators; thus, generator efficiency is potentially higher. Also, because of the intimate contact between liquid-metal and gas in the generator, the expansion process is nearly isothermal (an ‘infinite reheat’ turbine). Therefore, with proper heat regeneration, the cycle is a quasi-Ericsson cycle. The choice of working fluids may be either an ideal gas (e.g. helium, argon) or a condensible fluid (e.g. steam, CO₂).This paper relates the potential loss mechanisms in the LMMHD generator to various fluid dynamic phenomena that are discussed. Among these are effects resulting from velocity slip, vorticity generation and suppression by the magnetic field, wall friction, and shunt currents in liquid boundary layers. Experimental data relating to the internal flow phenomena and to overall generator performance are shown.The relative importance of these effects is discussed in relation to experimental evidence and known theory. It is concluded that the slip loss, resulting from a churn-turbulent flow pattern, is the major source of loss. A proposed means of creating a stable, homogeneous foam flow with virtually no slip, using the surface active properties of liquid metals, is discussed. Means are also suggested for reduction of wall shunt and frictional losses. The effects of end losses in two-phase flows are also described.     

Theoretical analysis of ammonia-water absorption cycles for refrigeration and space conditioning systems

This paper presents an investigation of an ammonia-water absorption cycle for solar refrigeration, airconditioning and heat pump operations at higher heat supply temperatures. The system consists of a solar driven generator, rectifier, condenser, evaporator, absorber and heat exchangers for preheating and subcooling within the system. A steady state thermodynamic cycle analysis based on mass and heat balances along with the state equations for the thermodynamic properties of the ammonia-water mixture has been carried out. A numerical computer simulation of the system with input component temperatures, refrigerant concentration/mass flow rate and effectiveness of the heat exchangers has been made to evaluate the relative heat transfer rates (i.e. coefficients of performance) and the mass flow rates for the cooling/heating modes. It is found that unlike the low generator temperature behaviour the coefficients of performance for both cooling and heating modes are reduced at higher generator temperatures. However, an increase of condenser temperature for each mode of operation improves the performance of the systems at higher generator temperatures. A choice for keeping the absorber temperature equal to/lower than that of the condenser is also predicted at lower/higher generator temperatures, respectively. In general the results are more pronounced for the refrigeration mode than for the heat pump mode and are least effective for the airconditioning mode.     

Thermodynamic analysis of the reverse Joule–Brayton cycle heat pump for domestic heating

The paper presents an analysis of the effects of irreversibility on the performance of a reverse Joule–Brayton cycle heat pump for domestic heating applications. Both the simple and recuperated (regenerative) cycle are considered at a variety of operating conditions corresponding to traditional (radiator) heating systems and low-temperature underfloor heating. For conditions representative of typical central heating in the UK, the simple cycle has a low work ratio and so very high compression and expansion efficiencies and low pressure losses are required to obtain a worthwhile COP. An approximate analysis suggests that these low loss levels would not necessarily be impossible to achieve, but further investigation is required, particularly regarding irreversible heat transfer to and from cylinder walls. In principle, recuperation improves the cycle work ratio, thereby making it less susceptible to losses, but in practice this advantage is compromised when realistic values of recuperator effectiveness are considered.     

Thermal efficiency improvement in high output diesel engines a comparison of a Rankine cycle with turbo-compounding

Thermal management, in particular, heat recovery and utilisation in internal combustion engines result in improved fuel economy, reduced emissions, fast warm up and optimized cylinder head temperatures. turbo-compounding is a heat recovery technique that has been successfully used in medium and large scale engines. Heat recovery to a secondary fluid and expansion is used in large scale engines, such as in power plants in the form of heat recovery steam generators (HRSG) [1]. The present paper presents a thermodynamic analysis of turbo-compounding and heat recovery and utilisation through a fluid power cycle, a technique that is also applicable to medium and small scale engines. In a fluid power cycle, the working fluid is stored in a reservoir and expanded subsequently. The reservoir acts as an energy buffer that improves the overall efficiency, significantly. This paper highlights the relative advantage of exhaust heat secondary power cycles over turbo-compounding with the aid of MATLAB based QSS Toolbox [2] simulation results. Steam has been selected as the working fluid in this work for its superior heat capacity over organic fluids and gases.     

Performance analysis of phase-change material storage unit for both heating and cooling of buildings

Utilisation of solar energy and the night ambient (cool) temperatures are the passive ways of heating and cooling of buildings. Intermittent and time-dependent nature of these sources makes thermal energy storage vital for efficient and continuous operation of these heating and cooling techniques. Latent heat thermal energy storage by phase-change materials (PCMs) is preferred over other storage techniques due to its high-energy storage density and isothermal storage process. The current study was aimed to evaluate the performance of the air-based PCM storage unit utilising solar energy and cool ambient night temperatures for comfort heating and cooling of a building in dry-cold and dry-hot climates. The performance of the studied PCM storage unit was maximised when the melting point of the PCM was ～29°C in summer and 21°C during winter season. The appropriate melting point was ～27.5°C for all-the-year-round performance. At lower melting points than 27.5°C, declination in the cooling capacity of the storage unit was more profound as compared to the improvement in the heating capacity. Also, it was concluded that the melting point of the PCM that provided maximum cooling during summer season could be used for winter heating also but not vice versa.     

Optimum design criteria for an Organic Rankine cycle using low-temperature geothermal heat sources

A cost-effective optimum design criterion for Organic Rankine power cycles utilizing low-temperature geothermal heat sources is presented. The ratio of the total heat exchanger area to net power output is used as the objective function and was optimized using the steepest descent method. Evaporation and condensation temperatures, geothermal and cooling water velocities are varied in the optimization method. The optimum cycle performance is evaluated and compared for working fluids that include ammonia, HCFC123, n-Pentane and PF5050. The optimization method converges to a unique solution for specific values of evaporation and condensation temperatures and geothermal and cooling water velocities. The choice of working fluid can be greatly affect the objective function which is a measure of power plant cost and in some instances the difference could be more than twice. Ammonia has minimum objective function and maximum geothermal water utilization, but not necessarily maximum cycle efficiency. Exergy analysis shows that efficiency of the ammonia cycle has been largely compromised in the optimization process than that of other working fluids. The fluids, HCFC 123 and n-Pentane, have better performance than PF 5050, although the latter has most preferable physical and chemical characteristics compared to other fluids considered.     

Design and thermodynamic analysis of supercritical CO2 reheating recompression Brayton cycle coupled with lead‐based reactor

A reheating process is generally incorporated in a supercritical CO₂ (S‐CO₂) Brayton cycle to enhance its efficiency. The heat transfer process from the reactor coolant to the working fluid of the power cycle is a key issue encountered when designing reheating power systems for the lead‐based reactor. The traditional reheating system, called RH‐1, utilizes an intermediate coolant circuit. In this paper, a novel reheating system, called RH‐2, is proposed. It eliminates the intermediate coolant circuit and combines the processes of the primary heating and reheating in a single heat exchanger. A thermodynamic analysis of three different systems for the lead‐based reactor integrated with the S‐CO₂ power cycle with or without reheating was conducted to evaluate the performance of the proposed system. The results confirmed that the performance of RH‐2 was the best of all the three systems. Under the same reactor conditions, the system efficiency of RH‐2 was greater than those of RH‐1 and the recompression (no reheating) system by 1.2% and 1.7%, respectively. RH‐2 could also maintain higher efficiency when the main operating parameters varied. The efficiency of RH‐2 was higher at different core outlet temperatures and split ratios. The maximum efficiency at optimal maximum pressure of RH‐2 was greater than those of the other two systems. RH‐2 was less sensitive to the variations in the isentropic efficiencies of the components than the other two systems, while the turbine isentropic efficiency demonstrated a significantly higher impact on the system efficiency than the two compressors (approximately 3.8 times).     

Low-temperature microwave-driven thermochemical generation of hydrogen from steam reforming of alcohols over magnetite

In a previous study, we reported that hydrogen generation can be achieved quite efficiently from seawater and contaminated waters at low temperatures using a microwave-driven hydrogen production (MDHP) process and waste activated carbon as the microwave absorption heating element (MAHE). However, the problem with this method is that activated carbon (a heat source from the microwaves) is used in the decomposition of water in which the quantity of activated carbon continues to decrease in parallel with the evolution of hydrogen. This problem has been resolved in this study by using magnetite as a novel MAHE component at low temperatures; the energy-saving thermochemical steam reforming reaction was performed with a mixed water/ethanol solution, for which results showed a maximum hydrogen generation yield somewhat greater than 80%. No hydrogen evolved in the thermochemical steam reforming process upon heating the magnetite at 350 °C in a conventional electric furnace, in contrast to the case where hydrogen was generated in yields greater than 40% by heating at 350 °C with microwaves. The rate-determining energy required for hydrogen generation is evidently heat, as evidenced by the formation of microwave-generated microscopic high-temperature fields (hot spots) at an estimated temperature of ca. 760 °C at the magnetite surface, a result of microwave heterogeneous microscopic thermal effects (MHMEs), and this even if the average temperature of the magnetite bulk were 350 °C.