Analysis of staggered tube bundle heat transfer to vertical foam flow

摘要

In general heat transfer intensity between solid surface and coolant (fluid) depends on three main parameters: heat transfer coefficient, size of heat exchange surface and temperature difference between surface and fluid. Sometimes the last two parameters (surface size and temperature difference) are strictly limited due to the process or technological requirements, and only increase of heat transfer coefficient is allowed. Simplest way offering sufficient increase in heat transfer rate (heat transfer coefficient as well) is to go from the laminar fluid flow regime to the turbulent one by increasing flow velocity. In many cases it helps despite such disadvantages like more complicated fluid supply system, rise of fluid flow mass rate and growth of energy usage for pumping. But in some cases, for example, in space application, in nuclear engineering, etc. there is not allowed to use high flow velocity of coolant - gas (due to vibration danger) or to apply high mass rate of coolant - liquid (due to limitation concerning weight or mass). One of the possible solutions of that problem could be the usage of two-phase flow as a coolant. An idea to use such two-phase coolant for heat removal from the solid surface is not new. Boiling liquid (water especially), gas flow with liquid droplets and other two-phase systems are widely used for heat and mass transfer purposes in various industries like food, chemical, oil, etc. An application of such two-phase coolants has lot advantages; high value of heat transfer coefficient is one of the most important. Unfortunately nothing is ideal on the Earth. Restrictions on vibration, on coolant weight (or mass rate); necessity to generate two-phase flow separately from the heat removal place; requirements on very low coolant velocities and other constraints do not allow using such type of two-phase coolant for purposes which were mentioned above (space application especially). As a possible way out can be usage of the statically stable foam flow produced from gas (air) and surfactant solutions in liquid (water). Our previous investigations [J. Gylys, Hydrodynamics and Heat Transfer under the Cellular Foam Systems, Technologija, Kaunas, 1998] showed the solid advantages of that type of two-phase coolant, including high values of heat transfer coefficient (up to 1000 W/m~2 K and more), low flow velocities (less than 1.0 m/s), small coolant density (less than 4 kg/m~3), possibility to generate foam flow apart from the heat removal place, etc. This article is devoted to the experimental investigation of the staggered tube bundle heat transfer to the vertical upward and downward statically stable foam flow. The investigations were provided within the laminar regime of foam flow. The dependency of the tube bundle heat transfer on the foam flow velocity, flow direction and volumetric void fraction were analyzed. In addition to this, the influence of tube position in the bundle was investigated also. Investigation shows that the regularities of the tube bundle heat transfer to the vertical foam flow differ from the one-phase (gas or liquid) flow heat transfer peculiarities. It was showed that the heat transfer intensity of the staggered tube bundle to the foam flow is much higher (from 25 to 100 times) than that for the one-phase airflow under the same conditions (flow velocity). The results of the investigations were generalized using criterion equations, which can be applied for the calculation and design of the statically stable foam heat exchangers with the staggered tube bundles.