Modelling and simulations for analysing thermal performance enhancement in air ducts with cold surface and hot air

Abstract: The world today has large challenges in order to manage a changing climate and the consequences that the climate change has on the environment and human living standards. This climate change is largely affected by the emissions of greenhouse gases, which come from usage of fossil fuels. This is a global problem that will affect the whole world and cause an increase of mean global temperature, which would lead to drastically changes in the living environment for human beings. A large part of the use of fossil fuels is connected to electric energy production. In EU almost half of the electric energy production is based on combustion of fossil fuels like natural gas and coal. These types of energy production need to be phased out and the energy consumption needs to decrease.   With climate change as a background there is a development towards more sustainable households. Companies around the world today invest in developing products that are more environmentally friendly. Household appliance companies develop products that use less water and energy, and a company like ASKO appliances AB tries to equip their machines with a new type of drying system. This new system would mean that less energy is needed for the drying cycle and humid air would not flow out in the kitchen.   The new drying system uses an air channel mounted on the side of a machine where moist hot air passes through the channel. During the passage the hot air will exchange heat with a cold surface inside of the channel. This work is focused on finding an optimal geometry of the air channel that enhances heat transfer between hot air and a cold surface. Installing obstacles inside of the duct could alter the flow pattern, and therefore enhance heat transfer. The work is mostly computer based with simulations performed in software called COMSOL Multiphysics. The software is used to build a 3-D model, where different geometries of obstacles are placed inside of the air channel. Results from simulations are compared with results from experimental trials, thus validating the computer model. Fluid flow simulations are used to investigate the effects of heat transfer for different types of geometries and sizes of obstacles. Parameters like influence angle and obstacle distance are tested.   The study shows results that obstacles inside of an air channel enhance heat transfer between a fluid and a surface. V-shaped obstacles perform the best results in order to enhance heat exchange, this compared with other tested geometries like W-shaped, wave-shaped obstacles and geometry without obstacles.   Different influence angle and distance between obstacles affects heat transfer, the study indicates that influence angle has larger effects on heat transfer than obstacle distance.