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Frost formation is a common, often undesired phenomenon in heat exchanges such as air coolers. Thus, air coolers have to be defrosted periodically, causing significant energy consumption. For the design and optimization, prediction of defrosting by a CFD tool is desired. This paper presents a one-dimensional transient model approach suitable to be used as a zero-dimensional wall-function in CFD for modeling the defrost process at the fin and tube interfaces. In accordance to previous work a multi stage defrost model is introduced (e.g. [1, 2]). In the first instance the multi stage model is implemented and validated using MATLAB. The defrost process of a one-dimensional frost segment is investigated. Fixed boundary conditions are provided at the frost interfaces. The simulation results verify the plausibility of the designed model. The evaluation of the simulated defrost process shows the expected convergent behavior of the three-stage sequence.

To estimate the heat flowing into the workpiece in machining processes, an inverse algorithm based on the Conjugate Gradient Method (CGM) is proposed to estimate the unknown boundary heat flux. Outgoing from infrared temperature measurements the heat flowing into the work-piece for an orthogonal cut can be estimated. To increase convergence of the estimated solution, a sensitivity analysis of the direct problem is performed to determine the identifiability of the boundary heat flux on the measurement site. The proposed Fixed Identifiability Conjugate Gradient Method (FIX-CGM) computes a step size function considering the identifiability of the unknown boundary condition to minimize the objective function. In contrast, the CGM computes a scalar step size by integrating the difference between measured and calculated temperature over time. Results show that applying the FIX-CGM for a benchmark case with a step heat flux faster convergence, better accuracy and less sensitivity to noise are achieved.

Abstract. Presence of frost in air coolers reduces the performance and thus has to be removed frequently. A method commonly applied for frost removal is hot-gas defrosting. For the efficient design of this periodically performed operating sequence, a precise prediction of the process is essential. A CFD simulation is a capable tool for this evaluation. This paper presents an integration of a defrost model into a CFD solver of the software OpenFOAM. The multi-region properties implemented in the existing code are adapted for the metal fin and tube structure and the air flow channel. The extension of the solver for evaluating the frost layer is accomplished by an interfacial wall function which models the frost characteristics and performs the thermal coupling. The extended solver is tested in a simplified environment, focusing on a fin-tube section. The simulated course of the defrost process and the total defrost time are discussed.