Microgravity experiments on the Constrained Vapor Bubble heat exchanger, CVB, are being developed for the space station. Preparatory experimental equipment have been demonstrated in the earth's environment. The liquid pressure field and resulting fluid flow rates are measured optically by analyzing the naturally occurring interference fringes recorded through a microscope. Fluid flow rates and heat flow rates are measured independently. Since the maximum attainable axial heat flux increases with the cell width, systems based on low capillary pressure regions are emphasized. These systems require a microgravity environment. However, the heat transfer and stability characteristics of these high flux systems are unknown and need to be determined.