Effect of Height Difference on The Performance of Two-phase Thermosyphon Loop Used in Air-conditioning System
Two-phase thermosyphon loop, Height difference, Experiment, Partially liquid filled
Two-phase thermosyphon loops (TPTLs) are highly effective devices for spontaneously transferring heat through a relatively long distance. Therefore, TPTLs are extensively used in various fields, such as cooling of electronic components, light water reactors, etc. Recently, the TPTL has also been found to be an effective way to recover or transfer heat in air-conditioning systems for energy saving. A typical TPTL consists of an evaporator, a riser (gas tube), a condenser, and a downcomer (liquid tube), and the condenser is higher than the evaporator by a certain vertical distance. The TPTL is powered by the natural force --- gravity, which means the pressure drop in the cycle always equals to the liquid head caused by the density difference of the liquid in the downcomer and the vapor or vapor/liquid mixture in the riser. Therefore, the liquid head is a key factor affecting the circulation flow rate and energy performance of TPTLs. In the traditional applications, the liquid head is considered to be proportional to the height difference between the condenser and the evaporator based on the underlying assumption: the downcomer is fully liquid filled. According to that, the TPTL will perform better with a larger height difference. The conclusion may be correct in the cases with large temperature difference and heat flux, such as in the field of cooling of electronic and light water reactors. However, when the TPTL is used in air-conditioning system, which has quite small temperature difference and small heat flux, some special phenomena were observed: the liquid heat is lower than the height difference and the downcomer is partially liquid filled. That is largely different from the thermosyphons in traditional applications. What’s the thermodynamic mechanism of partially liquid filled in the downcomer? How to determine the liquid head and the height difference? These are the fundamental questions that required answers before using two-phase thermosyphon in air conditioner field. In this study, the thermodynamic mechanism of partially liquid filled in the downcomer is researched and the effect of height difference on the performance of TPTL is investigated theoretically and experimentally. Firstly, a visual experimental setup is established, and the performance of a water-water TPTL is measured when the height difference ranges from 0 m to 2.4 m. Based on it, the basic phenomena are observed and the thermodynamic mechanism is investigated. Secondly, a generalized distributed-parameter model is developed based on the conservations of momentum, energy, and mass, which can determine liquid head and overall performance simultaneously according to external conditions. The model is verified by experiments. Then the model is used to analyze the variation of liquid head, circulation flow rate, heat transfer rate, system pressure under different height differences. The results show that with the increase of the height difference, the liquid head rises continuously until remain stable. Therefore, the liquid head is less than the height difference in some cases. Consequently, with the increase of height difference, the circulation flow rate and thermal performance firstly increases then remains constant.
Effect of Height Difference on The Performance of Two phase Thermosyphon Loop Used in Air-conditioning system