Identification of the unstable portion of the boiling heat transfer curve for cyclohexane using closed loop temperature control

The boiling heat transfer curve for cyclohexane has been investigated from 354K to 1200K at atmospheric pressure. Over the temperature range investigated, the three boiling regimes: nucleate, unstable film, and stable film were observed. These regimes were investigated using a vertically oriented, resistively heated graphite plate. Closed-loop control was utilized to obtain data in the unstable film boiling region. A heat transfer model was developed to facilitate development of the closed-loop control system.;Nucleate boiling occurred in the range from somewhat above the boiling point (354K) to approximately 367K. The observed critical heat flux for departure from nucleate boiling was 253 kW/m;As a direct result of this work, the unstable portion of the film boiling curve has been characterized. Unstable boiling, which occurred in the temperature range from 367 to 564K, has been divided into two additional regions. The first is a narrow region from 367 to 368K. In this region, the closed-loop controller that was developed did not achieve an adequate level of control. Yet from characterization of the regions on either side, it is known that the heat transfer surface flux drops from 253 kW/m;Stable film boiling began at 564K, where the heat transfer surface flux increased and followed a quadratic growth. A relationship that incorporates the fluid properties as a function of temperature was also developed. In addition, at high heat fluxes and temperatures above 1000K, gas phase (homogeneous) decomposition of the cyclohexane had to be considered.;A nonlinear finite difference heat transfer model for the cyclohexane graphite system was developed and appropriate constitutive material properties incorporated. The model was developed to facilitate the design of the classical PID controller that was implemented on the process for characterization of the unstable portion of the boiling heat transfer curve. Analysis of the nonlinear system relied heavily on time domain simulation of the nonlinear model, both open and closed loop. The model was also used in data analysis. A simple steady-state analysis was used to estimate surface temperatures and to validate the nonlinear model. Calculated surface temperatures were used to construct a calibration curve for obtaining the actual surface from the measured surface temperature.