Frost-free chilling

When UK-based General Domestic Appliances wanted to develop a new line of frost-free refrigeration appliances, they turned to a Computational Fluid Dynamics package from Fluent to help out.

When UK-based General Domestic Appliances (GDA) wanted to develop a new line of frost-free refrigeration appliances, they turned to a Computational Fluid Dynamics package from Fluent to help out.

One of the main goals of the project was to design the refrigerators with improved energy performance, to cut operating costs. To reduce the energy demands of the units, two aspects of the airflow inside the refrigerators had to be optimised.

First, the maximum air flow rate had to be generated using the smallest possible fan. This would not only improve the efficiency, but would also make the unit run more quietly. Second, the fan(s) and other internals needed to be positioned in such a way that the airflow inside both the refrigerator and freezer units was distributed in the most efficient way.

Test rigs were constructed so that measurements could be made in parallel with the CFD simulations. The role of these rigs was to validate the results of the CFD simulations and carry out the airflow optimisation phase of the project.

The largest freezer studied was 1.8 meters high and had 9 baskets. Because the geometry of the freezer is very complicated, with small gaps between the food packs and baskets, a tetrahedral mesh was used. The results for pressure distribution indicated that the largest pressure losses were occurring below and behind the bottom basket. This result was validated by measurements on the test rig. After increasing the clearance between the baskets and inside walls, the simulation was repeated, and the total airflow rate of the freezer was found to increase considerably.

The model was also used to study the pack temperature distribution in the freezer. A steady-state simulation was performed for a case where the compressor was running 100% of the time, and a transient simulation was performed when the compressor was cycling on and off. The results for the steady-state case suggested that the top and bottom basket have the warmest pack temperature if the air is uniformly distributed in the freezer. When the compressor runs intermittently, however, the top basket has the warmest pack temperature.

In order to reduce the pack temperature near the top and bottom baskets, the simulations showed that more air should be introduced to these regions.

Since the development of the refrigerators, models of other appliances have been developed and these have also assisted in the product development process.

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