Heat transfer to sub- and supercritical water flowin a vertical tube at lowmass fluxes

Hartog, Wilmar den (2016) Heat transfer to sub- and supercritical water flowin a vertical tube at lowmass fluxes.

Abstract:Within the Sustainable Process Technology research group of the University of Twente several processes are investigated which are operated at supercritical water conditions (TÈ374.0°C and PÈ22.05 MPa [1]). For example, supercritical water desalination, supercritical water gasification both operating at lowmass fluxes [2]. Operating processes at these conditions is highly energy consuming and has direct influence on the operating costs. Therefore heat integration is essential in these processes. Literature [4] describes a lot of research about heat transfer at sub- and supercritical water at high mass fluxes (G>200 kg/m2s [4]) and in a turbulent flow regime. In previous research [5; 6] a correlation for the heat transfer coefficient is developed for lowmass fluxes (G<20 kg/m2s) in the (overall) laminar flow regime. This correlation is supported with experimental results at subcritical temperatures. In this research experiments were carried out in a vertical tube (OD of 33.40 mm, ID of 20.70mmand length of 1.54 m) with upward flow at sub- and supercritical conditions. Experiments were done with 3 different flow rates; 4.1 (G=3.2 kg/m2s), 8.0 (G=6.3 kg/m2s) and 11.3 kg/h (G=8.8 kg/m2s). Temperatures are measured axial direction and in radial direction at a specific position. To get axial and radial temperature profiles. For the radial temperature profile in the fluid a probe with multiple thermocouples is used and for the radial temperature profile in the wall multiple thermocouples where placed in the tube wall at two positions (55 cmand 100 cm fromthe inlet). With the temperatures results from the wall the heat flux and thi inner wall temperature can be calculated. The temperatures of the probe are used to estimate the fluid temperature. Finally the Heat Transfer Coefficient (HTC) is calculated from the heat flux and temperature difference between the wall and fluid. The results obtained at 300 bar and 55 cm are the most consistent. Below 350 C the results of the experimental HTC are within the error bar in agreement with the correlation and increase from 1 kW/m2.°C up to 2 kW/m2.°C with temperature. However the experimental HTCs are always above the values obtained from the correlation. Above 350°C the HTC increases exponentially up to the pseudocritical temperature, which is not the case for the correlation. Experimental and correlation results show an increasing HTC at an increasing flow rate. The measurements at 100 cm are more difficult to interpret. Probably as a result of an axial temperature gradient in the wall the radial temperature profile in the wall shows a maximum. Due to this it is impossible to separate the radial and axial heat flux. Estimation of the maximum and minimum radial heat flux is conducted but it results in HTCs, which deviate too much.
Item Type:Essay (Master)
Faculty:TNW: Science and Technology
Subject:52 mechanical engineering
Programme:Sustainable Energy Technology MSc (60443)
Link to this item:http://purl.utwente.nl/essays/69004
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