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Effect of the surface texture on the heat transfer coefficient in nucleate boiling of aqueous lithium bromide solution at low heat fluxes.

Number: pap. n. 43

Author(s) : MENHART F., NATZER S., RIEPL M., et al.

Summary

State-of-the-art single-effect hot water driven absorption chillers widely use tube bundle falling film heat exchangers for heat and mass transfer. These heat exchangers are proven in functionality, but offer some disadvantages concerning production costs and volumetric density. A promising approach to overcome these disadvantages is the use of a flooded pool as generator or evaporator. While tube bundle falling film heat exchangers demand the complete wetting of the heat exchanger to activate the heat exchange area, flooded pool generators are wetted completely or at least partly by the solution. That leads to a significantly lower demand in solution circulation flow rates, which positively affects the thermal efficiency of the sorption process. On the other hand, the quality of heat transfer must be ensured. Pool boiling (nucleate boiling) facilitates high heat transfer coefficients. Yet, higher temperature differences between heating surface and fluid, compared to film boiling/ flow boiling, are needed to activate the pool boiling process. In addition to that, a pool generator causes higher equilibrium solution temperatures at the same process pressure, because of the static solution pressure at the heater surface. This cannot be neglected in low absolute pressure applications as LiBr/water absorption processes. Due to that, higher driving temperatures are needed for pool generators, compared to falling film generators. That could limit the operation of such an absorption process for applications with lower driving temperature supply (e.g. solar thermal cooling). Within this work, the influence of heat exchanger surfaces is analysed. A way to reduce the needed wall superheat is the surface treatment of the heat exchanger. By changing the surface condition, the density and size of nucleation sites can be affected, and the wall superheat can be reduced. So far, there are only a few publications, although a high potential is expected. To demonstrate the influence, different surface textures (smooth, undulated, structured, sanded and a grid on the smooth plate) were investigated. It could be shown, that the surface texture has an important influence on the heat transfer coefficient. With the simple treatment of the surface, a reduction of the wall superheat temperatures by around 10 % could be achieved.

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Pages: 10 p.

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Details

  • Original title: Effect of the surface texture on the heat transfer coefficient in nucleate boiling of aqueous lithium bromide solution at low heat fluxes.
  • Record ID : 30014318
  • Languages: English
  • Source: International sorption heat pump conference (ISHPC2014), College Park, United States, March 31-April 2, 2014.
  • Publication date: 2014/03/02

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