Development of multi-zone heat exchanger model under frost and defrost conditions

Global warming is now recognized by the major part of the scientific community. An important issue is the carbon dioxide emissions. The residential area is one major impacting sector, accounting for 30% of the final energy consumed (in Europe). A solution to reduce heat losses due to ventilation is single room ventilation with heat recovery. The objective of this paper is to present the development of a model for the design of a heat exchanger implemented in this kind of unit. The proposed modelling is a multi-zone model with a moving boundary. It allows to consider completely dry, partially wet or completely wet regimes on the stale air side. The boundary between the different zones is determined by means of the exchanger wall surface temperature. This model can be extended to frost conditions by adding a new “frost zone”. The formation of a frost layer on the surface generates an additional thermal resistance as well as a fouling inside ducts. Consequently, it reduces the flow section and the air flow rate for a given fan’s rotational speed. The frost quantity may increase until it blocks the passage ways for exhaust air and stops completely the airflow. The aim of this paper is to introduce the new multi-zone model taking into account the frost formation as well as defrost phase and predicting the impact on the hydraulic and thermal performance. This model is validated on the basis of some experiments done on an actual air-air heat exchanger under frost conditions. These experiments also reveals the limitations of the model.