High pressure processing (HPP) is emerging as a promising technology to achieve enzyme and microbial inactivation of food without compromising the food quality. The technology is being used to process both solid and liquid foods. The required high pressure can be realized by two different methods. While large scale industrial systems use mass addition to a confined space to increase the system pressure, the desired pressure is attained in laboratory and pilot scale systems using either mass addition or piston motion. Even though pressure is relatively constant during the process, heat generation resulting from compression of the fluid, the so-called compression heating, gives rise to non-uniform temperature distribution in the system. The mode of pressure application also contributes to such non-uniform temperature distribution.

Numerical simulation based on a commercial computational fluid dynamics (CFD) code was used to study the diffusive and convective heat transfer during HPP. The governing equations (momentum and energy equations) were solved using the finite volume method. Different boundary conditions were used to accommodate the different modes of pressure application. The physical properties were kept constant, with the exception of density of the medium, which was related to pressure and temperature. The profiles of pressure, temperature, and velocity of the fluid (medium) were predicted for different fluids, such as oil and water.

The simulations reveal that pressure was uniformly distributed in the system for both modes of application, mass addition and piston motion. In piston motion mode, the heat transfer is dominated by diffusion or thermal conduction during compression. In mass addition mode, the beginning of compression, heating is dominated by convective heat transfer. This led to a highly non-uniform temperature distribution in the system. This non-uniformity of the temperature distribution must be considered and fully accounted for during scale-up of a laboratory or pilot scale system to an industrial scale system for either pressure application mode.