Investigation into using liquid crystal thermography as primary temperature measurement technique for obtaining local wall temperatures and heat transfer coefficients in tube-in-tube heat exchangers

Abstract

The feasibility of obtaining local wall temperatures by using liquid crystal thermography (LCT) in a counter-flow tube-in-tube heat exchanger was investigated. Local annulus-side heat transfer coefficients at the inlet and thermodynamically and hydrodynamically underdeveloped regions were also obtained while operating at steady conditions. The heat transfer coefficients of the tube-in-tube heat exchanger are, however, disputed in the literature, as conflicting sources are easily found. In most literature sources the problem is simplified by assuming constant heat transfer coefficients throughout the length of the heat exchanger and the boundary layer growth is generally ignored at inlet regions. Thermocouples pose practical problems when measuring temperatures in heat exchangers. LCT is investigated as alternative surface temperature measurement technique. This study aims to develop a methodology for directly measuring wall temperatures inside a tube-in-tube heat exchanger. These temperatures were further used to calculate local heat transfer coefficients. In this study, a 1m long tube-in-tube test section with an annulus diameter ratio of 0.54 (ratio of the inner wall of the annulus to its outer wall) was constructed, in which liquid crystal thermography was employed as an alternative wall temperature measurement technique to thermocouples. Temperature-sensitive paint was applied to the inner wall of the annulus in order to measure the wall temperatures non-intrusively. Complete temperature maps could be constructed for different thermal conditions which indicated differences of up to 10 °C in wall temperature at the inlet regions, which would have been difficult to capture with thermocouples. This study covered a total of nine different annular flow and thermal conditions for cooled and heated cases. The annular flow conditions ranged from laminar (Re = 1000) flow to fully turbulent flow (Re = 13 800). In general, the heat transfer coefficients were found not to be constant along the length of the heat exchanger. The averaged heat transfer coefficients at the inlet were compared with existing correlations in the literature for full-length heat exchangers and were found to be higher by an average of 44 % over the data presented. Uncertainties on the local heat transfer coefficient were found to be approximately 80% for the cooled annulus cases and 45% for a heated annulus. This was mostly due to the practical laboratory restrictions imposed by fluid temperature limits. It was found that liquid crystal thermography could be used successfully for directly measuring the wall temperatures of tube-in-tube heat exchangers with very low surface temperature uncertainties (0.03 °C). With the approach developed in this study, a method was found for determining local heat transfer coefficients without introducing wall thermocouples or any other disturbances in the passage of the annular fluid.