Investigation of thermal performance of an air-to-air thermosyphon heat exchanger using ε-NTU method

Abstract

Experimental and theoretical research has been carried out to investigate the thermal performance of an air-to-air thermosyphon heat exchanger. Many factors affect the thermal performance of thermosyphon heat exchangers including velocity and temperature of input air, type and filling ratio of the working fluid, and pipe material. The air-to-air thermosyphon heat exchanger has been designed, constructed and tested in a test rig under steady state conditions. The lengths of both the evaporator and condenser sections of the heat exchanger were 600 mm and its central adiabatic section had a length of 100 mm. The heat exchanger had 90 plate finned copper thermosyphons arranged in 6 rows. A test rig was constructed and developed wherein the heated air is recycled into the evaporator section of thermosyphon heat exchanger. The temperature across the evaporator section was varied in the range of 100–250 °C while the inlet temperature to condenser section was nearly constant 25 °C. Distilled water was used as the working fluid with a fill ratio of 60% of the evaporator section length. The air face velocity ranged from 0.5 to 5.5 m/s and the heat input into the evaporator section was varied between 18 and 72 kW using electric heating elements. A computer simulation program based on the effectiveness-NTU method was developed to estimate the outlet temperature by iteration as well as thermal performance of the thermosyphon heat exchanger. Also several experiments were carried out under different operating conditions by varying the parameters in order to determine and investigate their effect on the thermal performance of the thermosyphon heat exchanger. The overall effectiveness of the thermosyphon heat exchanger obtained from experiments varied between 37% and 65%. The experimental results showed the minimum effectiveness of the thermosyphon took place at C h = C c. Therefore, equal value of air face velocities in evaporator and condenser sections should be avoided. It was shown that the experimental results were close to those obtained from computer simulation and became better as the velocity increases. The results of this experiment can be used in industrial cases.