Investigation of the thermal resistance of a thermosiphon with improved coolant circulation and a microporous coating in an evaporator


Аuthors

Kuzma-Kichta Y. A.1*, Shtefanov Y. P.2, Prokopenko I. F.2, Ivanov N. S.1**, Zhukova N. N.3, Romanyak A. Y.1

1. National Research University “Moscow Power Engineering Institute”, 14, Krasnokazarmennaya str., Moscow, 111250 Russia
2. ,
3. OKB GIDROPRESS JSC, Podolsk, Russia

*e-mail: kuzma@itf.mpei.ac.ru
**e-mail: ivanovniks@mpei.ru

Abstract

The paper examines the effect of two methods of heat transfer intensification – the installation of an axial insert that improves the circulation of the coolant and the application of a microporous coating in the evaporator – on the thermal resistance of a thermosiphon powered by freon R410a. The experiments were carried out on an installation with a thermosiphon having an evaporator made of AISI 304 stainless steel (Ø18×1 mm, length 2 m) and a pipe-in-pipe condenser with water cooling, at thermal loads in the range of 500–4800 W/m2. The wall temperature of the evaporator and condenser was measured using K-type thermocouples installed in 0,3 m increments, and the uncertainty of heat transfer measurements does not exceed 15 %.

Experimental data on the heat transfer coefficient in an evaporator without modification are compared with calculations based on known dependencies for boiling in pipes: the formulas of Labuntsov, Kutepov-Sterman and Dakhin. The Kutepov-Sterman formula showed the smallest deviation from the experiment (31 %), while the Labuntsov formula and the Dakhin formula give deviations of 47 % and 60 %, respectively.

The effect of an axial insert in an evaporator, which is a tube with three pairs of holes Ø4 mm, through which the working medium is supplied to the active evaporation zone, is investigated. In the range of thermal loads of 500–4800 W/m2, the installation of the insert leads to an increase in the heat transfer coefficient in the evaporator up to 41 % compared to the basic design without the insert, which is explained by improved fluid supply and more uniform circulation.

To further reduce thermal resistance, it is proposed to apply a sintered microporous coating of stainless steel particles to the inner surface of the evaporator. The calculation of heat transfer during boiling in a coated evaporator is performed according to the formula Kuzma‑Kichty Yu.A., taking into account porosity, coating thickness, particle diameter and effective thermal conductivity. It is shown that at optimal coating parameters (porosity 60 %, thickness 40 microns, particle diameter 20 microns), the heat transfer coefficient in the evaporator can increase up to 3 times compared with a smooth surface at a thermal load of 4800 W/m2. The obtained results demonstrate the prospects of a combined approach – improving the circulation of the coolant and applying a microporous coating – to significantly reduce the thermal resistance of the thermosiphon and increase its efficiency.

Keywords:

thermosiphon, heat transfer at boiling, heat transfer intensification, coating

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