Abstract
Two-phase closed thermosyphons (TPCTs) are widely utilized as heat exchanger elements in waste heat recovery systems and as passive heating/cooling devices. They are popular because of their high thermal conductivity, simple construction and reliability. Previous researches indicate that refrigerants are performing better than typical TPCT working fluids like deionized water or alcohols in the low temperature range. In the present study three HFC (Hydrofluorocarbons) refrigerants were tested: R134a, R404A and R407C. The total length of the investigated TPCT is 550 mm with equal length (245 mm) condenser and evaporator sections. Its outer diameter is 22 mm with 1 mm wall thickness. The evaporator section was heated by hot water with varying inlet temperature by 5 K step in the range of 288 K – 323 K. The condenser was cooled by cold water with inlet temperature kept at a constant value of 283 K. It was found that using R134a and R404A as working fluids heat transfer rates are the highest. For both refrigerants 10% is optimal filling ratio. They can be utilized interchangeably because the differences between their throughputs are within uncertainty bands. R407C performance was 50% lower. Other disadvantages of using this refrigerant are relatively high working pressures and higher optimal filling ratio (30%).
Highlights
Two-phase closed thermosyphons (TPCTs) known as wickless heat pipes are high thermal conductivity devices
The purpose of this work is to test the potential of the utilization of modern refrigerants as vertical TPCT working fluids
There is an obvious advantage of 10% filling ratio. 40% FR seems to be the worst as heat transfer rates are very small for low temperature differences between the evaporator and condenser
Summary
TPCTs known as wickless heat pipes are high thermal conductivity devices. Their construction is very simple – they are tightly sealed closed metal pipes partially filled with working fluid. TPCTs are utilized as elements of recuperative heat exchangers, cooling and temperature stabilization systems. Recuperative applications are characterized by a low temperature difference required for heat flow through TPCTs. In the design process of waste heat recovery systems, an essential aspect is a choice of working fluid and volumetric filling ratio. Pipatpaiboon et al [1] obtained experimentally thermosyphon heat exchanger throughputs for three working fluids: distilled water, methanol and R134a. For low temperature difference range R134 a exhibited higher thermal effectiveness than
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