Criteria for the absence of thermal convection in photochemical systems

The problem of avoiding thermal convection in a photochemically reacting fluid is considered. On the basis of an understanding of the light absorption, heat generation and transfer, and fluid mechanical processes occurring in a light-absorbing fluid, design criteria are developed that are sufficient to ensure the absence of thermal convection in photochemical experiments. The reaction cell should have a horizontal top and bottom and vertical, insulating side walls. The light entering the cell should be directed either vertically upward or downward and should fill the entire cross section of the cell. A simple equation is developed which gives the minimum temperature difference that must be imposed between the top and bottom boundaries in order to ensure a gravitationally stable density distribution. It is shown that the required temperature difference will be smaller if the illumination is from the bottom rather than the top. Finally, the results are applied to a recent experiment, showing that the criteria can easily be satisfied in practice.