The Performance of the First Pilot Thermoacoustic Refrigerator

The simple goal of this work is constructing a cheap, demonstrative model of a thermoacoustic refrigerator. To this end, the author succeeded in designing, building and testing the first pilot thermoacoustic refrigeration in Jordan basing on the theory of using sound waves as a coolant. The pilot thermoacoustic refrigerator was built from inexpensive and readily available parts in Mutah University, Jordan. The thermoacoustic refrigerator was operated for several hours. Consequentially, this experiment proved that thermoacoustic refrigerators were technically possible. Additionally, this experiment did yield some findings regarding the efficiency of thermoacoustic refrigeration. On other hand, solid flow CFD software was used to simulate the performance of thermoacoustic refrigerator especially the temperature and velocity inside the refrigerator. In general, very good agreement was deduced.     

Numerical and experimental study of a looped travelling‐wave thermoacoustic electric generator for low‐grade heat recovery

Thermoacoustic technology has drawn increasing attention due to its advantages such as reliability and environmental benignity. Aiming at low‐grade heat recovery, we developed a travelling‐wave thermoacoustic electric generator consisting of a looped travelling‐wave thermoacoustic engine and a linear alternator. In order to explore the operating characteristics of the electric generator, we numerically analyzed the acoustic field characteristics with a modified model. The analysis shows that high acoustic impedance appears in all three stages, and the travelling‐wave component dominates the acoustic field of the loop, which is significant for both thermoacoustic conversion and acoustic power propagation. Furthermore, we also investigated the effects of external electric compliance, resistance, and hot end temperature on the output electric power, thermal‐electric efficiency, and other related parameters. In the experiments, a thermal‐electric efficiency of 3.7% with an output electric power of 24 W has been achieved, when the hot end temperature is 120°C. The relative Carnot efficiency can exceed 14% when the hot end temperature is between 120°C and 190°C. The promising results demonstrate the significant potential of thermoacoustic electric generation in low‐grade heat recovery.     

Fundamental Study of a Loop-Tube-Type Thermoacoustic Heating System

This paper proposes a loop-tube type thermoacoustic heating system without any moving parts based on the thermoacoustic effect. In a thermoacoustic heating system, the supplied sound is converted to heat and the heating point is heated. A thermoacoustic heating system differs from a thermoacoustic cooling system： The location of the reference temperature section at the heat pump is upside down. The authors construct a prototype ofthermoacoustic heating system. The experimental results show that the heating point reaches 100 ~C. It must be emphasized that, using this simple and inexpensive thermoacoustic heating system, noise, waste heat and unused heat are useful as a renewable energy source.     

A probabilistic fouling and cost model for plate‐and‐frame heat exchangers

Fouling in plate‐and‐frame heat exchangers (PHEs) may be defined as the deposition of unwanted material on the heat transfer surface that reduces heat‐transfer and increases the resistance to fluid flow. Once the thermal–hydraulic performance decreases to a minimum acceptable level, cleaning of the equipment has to be done to restore the performance. The decision regarding periodic off‐line maintenance of the heat exchangers is generally based on a thermal–economic performance of the process. In this paper, we discuss a probabilistic maintenance model for PHE by incorporating the risk level and scatter parameter of the four random fouling growth models, namely linear, power law, falling rate and asymptotic models, which are integrated in the dimensionless cost model for a heat exchanger used in a steel plant. All the results are presented in terms of non‐dimensional plots. The results show that there is a strong relationship between t_down and the uptime, particularly in the region where the costs of operation and maintenance are minimum. Copyright © 2005 John Wiley & Sons, Ltd.     

Design optimization of heat exchangers with high‐viscosity fluids

The thermal effectiveness and entropy generation of parallel and counter‐flow heat exchangers handling high‐viscosity fluids have been numerically investigated. Both the viscous friction and the viscosity variations with temperature were considered in the analysis. The results show that the thermal effectiveness–NTU curves deviate gradually from the curves obtained using the assumption that the effect of viscosity is negligible. Moreover, the consideration of the viscous frictional heating effect results in a considerable increase in the heat exchanger entropy. An optimum heat exchanger size could be determined from both first law and second law of thermodynamics points of view. The results show also that the effect of viscous friction with variable viscosity becomes more significant for lower inlet temperatures of high‐viscosity fluid. Copyright © 2002 John Wiley & Sons, Ltd.     

Maximum entropy generation of in‐series connected heat exchangers

Theoretical analysis of entropy generation and availability destruction of “n” similar cocurrent or counter‐current heat exchangers connected in series are presented. A criterion for comparing the relative performance of any number of in‐series connected similar heat exchangers is developed. The effect of different influencing parameters such as the number of connected heat exchangers, the individual effectiveness of each unit, the heat capacity rate ratio and flow arrangement on the quality of heat exchange are presented. It is found that, the maximum of availability destruction (maximum entropy generation) for in‐series‐connected similar cocurrent heat exchangers is obtained at ϵ^*=1/(1+δ). However, for counter‐current heat exchangers connected in‐series; ϵ=1/(∑δ^i/n). This analysis might be useful for a proper choice of the number of heat exchangers to be connected together and the choice for the best operating conditions. Copyright © 2000 John Wiley & Sons, Ltd.     

Ground source heat pump––description and preliminary results of the Eco House system

A reversible ground source heat pump has been installed and tested at the Eco House, University of Nottingham. Results obtained during experimental testing are presented and discussed here. Various loop designs were considered including a tank coil system that aids the heat transfer process via rainwater drainage. The results show that the design coefficient of performance (COP) of a heat pump unit can be achieved through proper installation and careful consideration of the operation of the system as a whole.     

Contribution to the Study of the Reduction of Energy Consumption through the Exchanger Coupled Conventional Air‐Ground‐Air Conditioner. Application to the Building

In this study, a new method of using the earth‐air heat exchangers to reduce energy consumption in buildings is discussed. The idea is to couple the EAHE with the condenser of a residential air conditioning system to enhance the effectiveness of the latter. Under the climatic conditions of high temperature in summer (south‐eastern region of Algeria), which can sometimes exceed 50 °C, what makes the heat exchange between the air conditioner and the external environment very difficult in addition to the problem of thermal comfort and the cost of energy consumption. Conducting a simulation by the TRNSYS software that allows to couple the model of the EAHE with the condenser of a residential air conditioner and connect the system with a building. The results show a clear reduction in the energy consumed by this system in connection with the direct use of the air conditioner and increase air conditioning efficiency, coefficient of performance, and energy efficiency rating. In this paper the Hollmuller model was ameliorated and the obtained results are in concord with it. The system is capable of resolving the problem of mal cooling of buildings by air conditioners under critical climate conditions, in addition to lowering the heat output of the condenser, and reducing its effect on the environment.     

Theoretical thermal performance analysis of two solar‐assisted heat‐pump systems

The thermal performance of two different schemes of solar‐assisted heat‐pump systems has been theoretically studied. In first scheme, the evaporator of the heat pump is taken directly as the solar collecting plate and always maintained at the ambient temperature. As there is no heat loss from the collecting plate, the thermal efficiency of the collector is high and equals the solar absorptivity of the collecting plate. As suggested, the heat‐pump evaporator of the second scheme is placed in a novel fresh water solar pond/tank with high efficiency. Since the evaporator operates at a relatively high temperature, the COP of the heat pump can be increased. The calculated results show that the COP of a solar‐assisted heat pump using the second scheme is considerably higher than that of the first scheme. Copyright © 1999 John Wiley & Sons, Ltd.     

On the steady‐state modelling of a two‐stage evaporator system

We develop and validate against experimental measurements a steady‐state two‐stage flooded refrigerant evaporator model for a heat pump drying system. A prototype two‐stage heat pump dryer test facility was designed, built and instrumented to provide the required measurements for the validation of the model. Repeatability and data quality tests were conducted to evaluate the accuracy of measurements. Experimental data could be reproduced to within ±6.5 per cent of replicated air and refrigerant side measurements for the same evaporator's air inlet conditions while the discrepancy of energy balance at the air‐side and refrigerant‐side was observed to be within ±8.9 per cent. The two‐stage evaporator model predicted the air‐side total heat and latent heat transfer of the two‐stage evaporator to within (?6.3 per cent, 7.6 per cent) and (?11.5 per cent, 9.5 per cent), respectively. On the refrigerant‐side, the model enabled the calculation of the degree of superheat to within (?10.6 per cent, 1.7 per cent). The model has shown that there is significant improvement in the heat recovered from a two‐stage evaporator system compared to a single evaporator system. In addition, the model demonstrated that the improvement in total heat recovery could be as high as 40 per cent over its base‐value when the latent to total load at the two‐stage evaporator is increased. Copyright © 2001 John Wiley & Sons, Ltd.     

Performance of a spiral ground heat exchanger for heat pump application

A high-efficiency ground heat exchanger has been developed for use with ground-source heat pumps. The exchanger is made of copper tubing, shaped in the form of a spiral, which can be installed in a vertical borehole backfilled with sand. Thermal performance of a full-scale prototype indicated that this heat exchanger can achieve very high heat extraction rates if subfreezing operating temperatures are used. For most soil types cyclic freezing and thawing is not a problem; however, for the sensitive Leda clay in which the prototype tests were conducted, substantial settlement occurred after the first freeze-thaw cycle owing to initial collapse of the soil structure.     

涡轮叶片尾缘扰流柱最佳形状的研究

以涡轮叶片尾缘中扰流柱的换热为应用背景，讨论了一定条件下获得最大换热量时扰流柱的形状曲线，系统分析了这种最佳曲线随扰流柱物性参数及几何特征变化的规律。     

Thermodynamic optimization of heat‐transfer equipment configuration in an environmental control system

In this paper, we show that many features of a heat transfer installation can be deduced from the maximization of the global performance of the greater system that employs the installation. The heat transfer installation is a series of two cross‐flow heat exchangers. The greater system is the environmental control system (ECS) of an aircraft. The global performance objective is the minimization of the total thermodynamic irreversibility of the ECS. Several architectural features are deduced from principle: the relative position of the two heat exchangers, their relative sizes, and all the geometric aspect ratios of the two heat exchanger cores. We find that the optimized architecture is insensitive (robust) to changes in some of the external parameters. Robustness is a useful feature because it simplifies the design work. Furthermore, one design that is built can be expected to function at near‐optimal levels when the external parameters change. The application of this method of topology optimization to more complex systems is discussed. Copyright © 2001 John Wiley & Sons, Ltd.     

Experimental Investigation of Heat Transfer Characteristics and Cooling Performance of Mini‐ and Microheat Exchangers

This paper presents an experimental investigation of forced convection heat transfer in two heat sinks for electronic system cooling and investigated the comparisons of the thermal behavior of the mini‐ and microchannel heat exchangers. The hydraulic dimension of one of the heat sinks is 2 mm while that of the other is . Deionized water was used as the working fluid for studies conducted in both the heat exchangers. The effect of heat flux and volumetric flow rate (in laminar flow regime) on temperature and heat transfer coefficient is studied. Irrespective of the average heat transfer coefficient and the total thermal resistance, advantages and limitations of each device are analyzed and discussed in the light of experimental results. Furthermore, the results obtained from the experiments were in good agreement with those obtained from the design theory analyses.     

热电(冷)联产系统的优化性能

依据有限时间热力学原理导出了不可逆热电联产和热电(冷)联产系统在系统最大火用输出时的基本优化关系,确定了热电(冷)联产系统优化参数和优化构形选取范围,得到了供热(制冷)和发电间的匹配优化特性,通过数值算例得出不同参数对系统性能影响的规律。所得结论可为热电(冷)联产系统的优化设计和最佳工况选择等提供理论依据。     

Evaluation of PMSG‐based drivetrain technologies for 10‐MW floating offshore wind turbines: Pros and cons in a life cycle perspective

This paper presents an in depth evaluation and comparison of three different drivetrain choices based on permanent‐magnet synchronous generator (PMSG) technology for 10‐MW offshore wind turbines. The life cycle approach is suggested to evaluate the performance of the different under consideration drivetrain topologies. Furthermore, the design of the drivetrain is studied through optimized designs for the generator and gearbox. The proposed drivetrain analytical optimization approach supported by numerical simulations shows that application of gearbox in 10‐MW offshore wind turbines can help to reduce weight, raw material cost, and size and simultaneously improve the efficiency. The possibility of resonance with the first torsional natural frequency of drivetrain for the different designed drivetrain systems, the influence of gear ratio, and the feasibility of the application for a spar floating platform are also discussed. This study gives evidence on how gearbox can mitigate the torque oscillation consequences on the other components and how the latter can influence the reliability of drivetrain.     

Development of a four‐cell DMFC stack with air‐breathing cathode and dendrite flow field

A four‐cell direct methanol fuel cell (DMFC) stack with an air‐breathing cathode with an active area of 0.48 cm² for each cell is designed, fabricated and tested. A pure copper sheet 300 µm thick with innovative perforated flow plates (dendrite type) is fabricated and used for the cathode. For the anode, conventional serpentine flow channels made of pure copper sheets 250 µm thick are used. An extensive parametric study is conducted to determine the optimum working conditions for the fuel flow rate (anode), methanol solution concentration, channel‐to‐land ratio and stack temperature. Comparisons are made with conventional serpentine flow channels. In addition, CO₂ (water) bubbles in the anode (cathode) channels are visualized, and the results are presented and discussed. It is found that the maximum stack power of the four‐cell μDMFC stack is up to 40 mW/cm² with a limiting current density of 335 mA/cm² at a maximum volumetric and gravimetric power density of 11.16 mW/cm³ and 3.13 W/kg, respectively. Copyright © 2014 John Wiley & Sons, Ltd.     

High‐pressure hydrogen storage properties of TixCr1 − yFeyMn1.0 alloys

Ti_xCr_{1 ? y}Fe_yMn_1.0 (x = 1.02, 1.05, 1.1, 0.05 ≤ y ≤ 0.25) alloys were prepared by plasma arc melting and annealing at 1273 K for 2 hours. The XRD results show that the main phase of all alloys is the C14 type Laves phase, and a little secondary phase exists in a mixture of the binary alloy phase. The lattice parameters increase with Ti super‐stoichiometry ratio increasing, whereas smaller lattice parameters emerge with increasing Fe stoichiometry content. Additionally, as the Ti super‐stoichiometry ratio decreases, the pressure‐composition‐temperature measurements indicated that hydrogen absorption and desorption plateau pressures of Ti_xCr_0.9Fe_0.1Mn_1.0 (x = 1.1, 1.05, 1.02) alloys increase from 3.15, 0.67, to 5.94, 1.13 MPa at 233 K, respectively. On the other hand, with the Fe content increasing in Ti_1.05Cr_{1 ? y}Fe_yMn_1.0 (0.1 ≤ y ≤ 0.25) alloys from 0.1 to 0.25, the hydrogen desorption plateau pressures increased from 1.41 to 2.46 MPa at 243 K. The hydrogen desorption plateau slopes reduce to 0.2 with Ti super‐stoichiometry ratio decreasing to 1.02, but the alloys are very difficult to activate for hydrogen absorption and cannot activate when the Fe substituting for Cr exceeds 0.2. The maximum hydrogen storage capacities were more than 1.85 wt% at 201 K. The reversible hydrogen storage capacities can remain more than 1.55 wt% at 271 K. The enthalpy and entropy for all hydride dehydrogenation are in the range of 21.0 to 25.5 kJ/mol H₂ and 116 to 122 J mol^?1 K^?1, respectively. Our results suggest that Ti_1.05Cr_0.75Fe_0.25Mn_1.0 alloy with low enthalpy holds great promise for a high hydrogen pressure hybrid tank in a hydrogen refueling station (45 MPa at 333 K), and the other alloys of low cost may be used for a potable hybrid tank due to high dissociation pressure at 243 K and high volumetric density exceeding 40 kg/m³.     

Power generation characteristics of a sandwich‐type self‐heating thermoelectric generator with spatially varying embedded heat source

Power generation characteristics of a sandwich‐type thermoelectric generator in which the heat source is embedded into thermoelectric elements are investigated. Our previous work on a similar concept only considered a uniform heat source distribution inside thermoelectric elements. In this work, the effect of the spatial distribution of a heat source is examined. In particular, the effect of the concentration of heat source near the one end, that is, the hot end, is intensively studied as a potential means of improving the efficiency of the device. Although the effects of heat source concentration in impractical cases without heat transfer limitations on the cold side remain ambiguous, it become clear that heat source concentration indeed has positive effects in more realistic cases with finite heat transfer coefficients imposed on the cold side. Because of the relatively low efficiency of typical thermoelectric generation, a significant amount of heat must be dissipated from the cold end of the thermoelectric element. Greater heat source concentration near the hot end leads to more effective utilization of available heat source, reduces the amount of heat rejected at the cold end, and lowers the hot end temperature of the thermoelectric element. Overall, it is suggested that heat source concentration can be used as a method to achieve more efficient operation and better structural integrity of the system. Copyright © 2015 John Wiley & Sons, Ltd.