Evaluation of heat sink capability and deposition propensity of supercritical endothermic fuels in a minichannel

The aim of this study was to develop approaches for the evaluation of heat sink capabilities and deposition propensities of different endothermic fuels in a minichannel. It should be noted that compared with our previous paper [23] which proposed the hydraulic resistance method (HRM) and validated it using one fuel pyrolyzed at different fluid temperatures, the significant one of the innovative points of this paper is that the HRM was corrected and applied to evaluate five different hydrocarbons, which are three pure hydrocarbons (cyclohexane, n-hexane and toluene), one petroleum-derived fuel and one synthetic fuel that were modeled as endothermic fuels. All of these fuels were tested in a minichannel with an internal diameter of 2.0 mm. The qualities of high volumetric heat sink capacity and good anti-coking capability were proven to be two contrary features in endothermic fuels. All of the five hydrocarbons tested had a mass heat sink of approximately 3.4 MJ/kg at a temperature of 750 degrees C and pressure of 5 MPa. However, the volumetric heat sinks (per unit volume hydrocarbon at the standard condition) were considerably different due to the hydrocarbons' varying densities. Overall, cyclohexane showed better performance than the other four hydrocarbons, as it had a relatively high volumetric heat sink and good anti-coking characteristics. The coking runs were designed for a steady state at a bulk fluid temperature of 750 degrees C for 20 min, which allowed measureable coke deposition layers to accumulate on the channel's inside surface. Another innovative point is that except the HRM, methods used to assess the deposition propensity included pressure drop changes during the coking run process, measurements of coke deposition in the filter, observations of the color features of liquid products and the weighing method, all of those methods are aided for the HRM to better differentiate the deposition propensity for different fuels. These different methods yielded a common conclusion that the coke deposit rates of the tested hydrocarbons could be ranked as follows: endothermic fuel > cyclohexane > n-hexane > toluene. It was found, however, that the deposit thickness was only one of the factors affecting heat transfer performance for different hydrocarbons, and the thickness was far from sufficient to reduce the heat transfer that led to the increased wall temperatures. (C) 2015 Elsevier Ltd. All rights reserved.