Instability of a two-phase loop thermosyphon

The instability of two-phase loop thermosyphons was investigated experimentally and analytically. Three orifice type inserts were used to study the effect of change in the pressure drop in the flow channel of the TLT on the flow instability and temperature fluctuation. It is observed that a decrease in the size of the orifice insert from 3.7 mm (no insert) to 0.71 mm drastically reduced the fluctuation of the temperature, especially at the evaporator section of the TLT. With the orifice type insert of 0.71 mm for the TLT, the overall temperature fluctuation was almost completely eliminated, especially at higher power input to the TLT. The analysis based on the Kelvin-Helmholtz instability theory seems to predict reasonable well the loop stability state of the TLT with experimentally determined constant factors.     

Thermal convection in a porous toroidal thermosyphon

Stability of natural convection in porous media differs from that in Newtonian fluid due to the changes of the thermal inertia and the friction. This paper aims to investigate the natural convection and its stability in a toroidal thermosyphon filled with porous media. The flow and temperature fields were numerically simulated and compared with that of Newtonian fluid. The results predicted two obvious local reversal flows near the connections of heating and cooling halves. A one-dimensional model was proposed which described qualitatively the occurring and stability of the thermal convection. The model suggested that the flow stability depends largely on the Prandtl number. The global flow can probably be chaotic only if the Prandtl number is finite. Contradictions of the conclusions drawn by different researchers were also discussed.     

Performance of large solar water heating system: Thermosyphon mode

Two large solar water heating systems (non-pressure type), each having 1000-1. capacity, have been installed at IIT New Delhi and their performances have been studied under the thermosyphon mode between the collectors and the storage tank. A simple transient analysis of the system, incorporating the effect of withdrawal of hot water from the storage tank, has been developed. The effect of stratification in the storage tank has been studied experimentally. It is found that the experimental observations are in good agreement with the theoretical results obtained by the present model.     

Experimental and analytical performance study of a thermosyphon water heater

The performance of a thermosyphon solar water heater was studied analytically and experimentally. A finite-difference model was used to predict year-round performance. Tests were conducted on an experimental heater subjected to acutal weather conditions in Benghazi, Libya. Satisfactory qualitative and quantitative agreement was found between experimental and predicted results. A storage volume of 60 liters per unit collector area was found to be optimum for Benghazi conditions. The day-end temperature was found to vary between 23° and 51°C for the test period, which occurred in winter, with an average of 41°C.     

Development of a heat exchanger for the cold side of a thermoelectric module

A heat exchanger for the cold side of Peltier pellets in thermoelectric refrigeration, based on the principle of a thermosyphon with phase change and capillary action has been developed. This device improves the thermal resistance between the cold side of a Peltier pellet and the refrigerated ambient by 37% (from 0.513 of the finned heat sink, to 0.323 K/W). Analytic calculations and experimental optimisation of the TPM have been carried out by building and testing several prototypes. It also has been experimentally proved that the COP of thermoelectric refrigerators can be improved up to 32% (from 0.297 to 0.393) by incorporating the developed device.     

Hybrid photovoltaic and thermal solar-collector designed for natural circulation of water

The electricity conversion-efficiency of a solar cell for commercial application is about 6–15%. More than 85% of the incoming solar energy is either reflected or absorbed as heat energy. Consequently, the working temperature of the solar cells increases considerably after prolonged operations and the cell’s efficiency drops significantly. The hybrid photovoltaic and thermal (PVT) collector technology using water as the coolant has been seen as a solution for improving the energy performance. Through good thermal-contact between the thermal absorber and the PV module, both the electrical efficiency and the thermal efficiency can be raised. Fin performance of the heat exchanger is one crucial factor in achieving a high overall energy yield. In this paper, the design developments of the PVT collectors are briefly reviewed. Our observation is that very few studies have been done on the PVT system adopting a flat-box absorber design. Accordingly, an aluminum-alloy flat-box type hybrid solar collector functioned as a thermosyphon system was constructed. While the system efficiencies did vary with the operating conditions, the test results indicated that the daily thermal efficiency could reach around 40% when the initial water-temperature in the system is the same as the daily mean ambient temperature.     

Studies on heat transfer and friction factor characteristics of thermosyphon solar water heating system with helical twisted tapes

Experimental investigation of heat transfer, friction factor and thermal performance of thermosyphon solar water heater system fitted with helical twisted tape of various twist ratios has been performed and presented. The helical twisted tape induces swirl flow inside the riser tubes, which increases the heat transfer and pressure drop. The empirical correlations developed for Nusselt number and friction factor with various twist ratios (Y = 3, 4, 5, 6) are fitted with the experimental data with a discrepancy of less than ±4.54% and ±6.13% respectively. The results are compared with a plain tube collector at the same operating conditions. Conclusions made from the results show that heat transfer enhancement in twisted tape collector is higher than the plain tube collector with minimum twist ratio and gradually decreases with increase in twist ratio. The overall thermal performance of twisted tape collector is found to increase with increase in solar intensity.     

两相闭式热虹吸管换热特性的数值模拟

本文建立数学模型对两相闭式热虹吸管换热特性进行数值模拟。考察了几何尺寸、工质及加热条件对液膜流态、液膜厚度等的影响,从而提示了这三个部因素影响冷凝段换热特性的内在机理。     

重力热管的工质选择

作为高效传热方式，重力热管已在太阳能技术、冻土保持等方面获得广泛应用。本文介绍了工质选择的准则，然后讨论工质的热物理特征对热管换热性能的影响。     

Analysing the effect of the laeh in the storage tank on the performance of a tsaeh

When an auxiliary electric heater (AEH) is incorporated in a thermosyphon solar water system, the system performance becomes more complex and it is more difficult to simulate and analyse its operational characteristics. Using a ‘fixed-node, equally-convecting-mixing model’ that they have developed, the authors perform detailed simulations and analysis of the effects of the location of the auxiliary electric heater (LAEH) in the storage tank on the performance of the thermosyphon solar water system with an auxiliary electric heater (TSAEH), and discover for the first time some new laws governing the system operation. The authors believe that their simulations and analysis will be very beneficial for optimal design and practical application of such systems in the future.