Direct contact heat transfer is generally defined as heat transfer between two or more mass streams without the presence of an intervening wall. The mass streams can be cocurrent, countercurrent or even crossflow. The streams can be immiscible or miscible or partly so. Typical two-stream direct contactors include: liquid–liquid, liquid–vapor, liquid–solid, gas–solid, or even solid–solid. Common systems that have been studied extensively include: water–air, water–steam and water–organic liquid. Also, there have been extensive studies of fuel drops in an oxidizing gaseous environment. The possibilities are legion.
Direct contact heat transfer can take place across the interface between two continuous fluid streams, such as a gas flowing over a thin liquid film or between a disperse spray and a gaseous or vapor stream into which the spray is injected. The former might involve hot gas quenching or fuel vaporization and combustion, while the latter might involve the condensation of the vapor on droplets within the spray. Still another example is the cooling of fine drops of a liquid undergoing solidification, as in the manufacture of glass beads or metal shot.
In many cases of direct-contact heat transfer, chemical reactions can be taking place between the mass streams, and one of the gas streams may totally be consumed by the other. There can, of course, be simple sensible heat transfer as in the case of two immiscible liquids.
When the mass streams include at least one fluid, that stream can be either laminar or turbulent; in many applications, turbulence needs to be avoided as it can lead to problems in either characterization of the mass streams or bulk fluid dynamical changes.
A further characteristic of direct contact processes is that the fluid streams must locally be at the same pressure. Many industrial direct contactors thus develop relative flows of the mass streams by imposing an external body force. The most common of these is the effect of gravity or centrifical forces on two fluids of different density, although electrical or magnetic fields can be imposed on some fluids to achieve the desired effect of relative fluid motion.
Direct contact heat transfer has received considerable attention since the 1970's, although the field lacks the maturity associated with other heat transfer. Specific reviews of direct contact heat transfer include those of Sideman (1966), Sideman and Moalem-Maron (1982), Jacobs (1988) and Jacobs (1995). Jacobs has defined direct-contact heat transfer as being associated with either continuous streams or dispersed phases. In the former, the heat transfer can be of steady-state while for the latter, the heat transfer is always transient when one considers the individual drops, bubbles or particles of which the dispersed phase may be composed. When one of the fluids is composed of a dispersed stream, the bulk flow may appear to be undergoing a steady-state energy transfer, as in the case of a spray column or a sieve tray column; however, the individual fluid elements are undergoing transient heating. It is thus necessary to carry out a combined Eulerian-Lagrangian analysis of the flow and heat transfer. It is this characteristic of direct-contact heat transfer that makes it more difficult to model than surface-type heat exchange. All of the difficulties of modeling multiphase flow are present, together with the complications of heat exchange, and define the interfacial phenomena. Nevertheless, the advantages of potentially much higher heat transfer rates, the ability to transfer the heat at much lower temperature differences between the streams and the potentially lower cost makes direct-contact heat transfer extremely attractive.
To learn more about direct contact heat transfer, refer to the literature reviews cited above and to the articles on Direct Contact Heat Exchangers, Dryers, Cooling Towers, Condensers, Quenchers, Multiphase Flow, etc.
Sideman, S. (1966) Direct Contact Heat Exchange Between Immiscible Liquids, Advances in Heat Transfer. Academic Press, New York, NY, 207-286.
Sideman, S. and Moalem-Maron, D. (1982) Direct Contact Condensation, Advances in Heat Transfer, Academic Press, New York, NY, 228-276.
Jacobs, H. R. (1988) Direct Contact Heat Transfer for Process Technologies, ASME Journal of Heat Transfer. Vol. 110, 1259-1270.
Jacobs, H. R. (1995) Direct Contact Heat Transfer. To be published in the Heat Exchanger Design Handbook (HEDH) Update, Begell Press.