Rotating heat pipe with Dowtherm A for intermediate temperature applications in a heat pipe assisted annealing process

Abstract

The production of steel is an energy intensive process, using 20GJ/tonne of steel. The energy use needs to be declined by 10% in 2030 in order to be on track of the Sustainable Development Scenario(SDS). About 1GJ/tonne is required today by the annealing process, a heat treatment in which the steel strip is heated to a temperature of about 600-700^oC and cooled afterwards. Tata Steel came up with a way of connecting the cooling and heating section in an innovative way, which can potentially reduce the energy requirements of an annealing line by up to 70%. Horizontal rotating heat pipes are proposed to transfer heat efficiently over its axial length from the strip in the cooling section to the strip in the heating section. Dowtherm A has been selected as the working fluid between 150 and 350^oC. No research is available on rotating heat pipes with Dowtherm A. The aim of this study is to research the relevant internal heat transfer characteristics of a
rotating heat pipe and gain insight into the performance of Dowtherm A as the working fluid in terms of heat transfer efficiency. Furthermore, the aim is to gain knowledge on the effect of non-condensable gas inside a heat pipe and on the way the effects can be modelled in a computationally efficient way. Non-condensable gas is likely to be of influence on heat transfer homogeneity outwards to the relatively cool steel strip. Understanding this influence is another goal of this study. To fulfil the aims of this study, experiments are conducted and a computational model is devised. An experimental setup with working fluid Dowtherm A is used to acquire data at different rotational speeds, operating temperatures and at different thermal inputs and outputs. The rotating heat pipe used is of smaller scale than one in a heat pipe assisted annealing line, but with comparable heat fluxes. Secondly, the devised model is used to both qualitatively and quantitatively study the effect of non-condensable gas for different operating
parameters and non-condensable gas amounts, which is done for conditions as in the experimental setup and as in a heat pipe assisted annealing line. The rotating heat pipe worked successfully during conducted experiments and Dowtherm A has shown to be able to transport at least 280W axially through the inner geometry in the non-annular flow regime, which is 192,000W/m² and corresponds to a vapor flow of 0.001kg/s, at an axial temperature difference of only a few degrees Celsius. Nucleate boiling was considered the dominant heat transfer mechanism through the film in the evaporator, with a typical heat transfer coefficient observed of 4200W/m²K. The condenser has shown a lower film heat transfer coefficient; 1750W/m²K. The effect of rotational speed, power and temperature is minor in the evaporator at conditions tested. The 1D model for determining non-condensable gas distribution showed good agreement with reported experimental data, which show a major axial temperature drop at the condenser end. It was shown that wall conduction influences non-condensable gas distribution for heat pipes with a relatively thick wall, which is modelled by the addition of wall temperature calculations to the 1D model. The effect of non-condensable gas on heat pipe performance showed to be strongly dependent on
operating temperature. Axial convective transport showed to be between two and three orders of magnitude more efficient than pure conduction. Homogeneous heat outflow is achieved when non-condensable gas is not present in the condenser, which can be achieved by extending the heat pipe at the condenser end. The devised model is suitable to predict the effect of operating conditions and design adjustments in an efficient way. Results have shown that Dowtherm A is a suitable working fluid for a heat pipe assisted annealing line, due to the low internal resistances to heat and mass transfer.