The Enhancement of Heat Transfer in Falling Film Evaporators by a Non-Uniform Flow Rate

Abstract

In an evaporator, with a falling film of uniform thickness and velocity, it is well known that the heat transfer coefficient h (w m^-2 K^-1) is a decreasing function of the fluide velocity in laminar regime and an increasing function in turbulent regime.

However, if the liquid is flowing in the form of rapid rivulets on some parts of the wall, and in the form of thinner films on other parts, we show that such a non-uniform distribution of the fluid velocities, produces an enhancement of the overall heat transfer coefficient h, by a ratio of the order of 2. The addition of vertical fins on the wall may have a double effect : increasing the evaporating surface area, and also increasing the non-uniformity of the liquid flow distribution.

The present research programme is a part of a contract joue concerning an “Absorption-driven four-effect evaporator” coupled with a two-effect regenerator under the direction of Prof. S. Yanniotis and the company “hellas energy”.

We will show that, in a falling film evaporator, a non-uniform distribution of the thickness of the falling film induces an enhancement of the heat transfert coefficient.

The non-uniformity of flow may be obtained either in a spatial geometric structure (corrugated wall surface, adjunction of metallic grids ...) or in a temporal structure (periodic waves).

We consider the case of a liquid film being evaporated as it flows down a vertical heated surface. First let us assume that the flow is perfectly uniform. In other words the film has the same thickness and velocity at all points at the same level. Theoretical and experimental results in the literature show that the heat transfer conductance h between the surface and the liquid-vapour interface versus flow rate decreases in the laminar regime, passes through a minimum and then rises in the turbulent regime. The decrease in h in the laminar regime is due the progressive increase in the film thickness. Then, at a certain critical flow rate waves start to form in the film (wavy laminar regime). As flow rate is increased these waves are replaced by eddies which become more and more intense as flow rate is increased. These eddies increase the value of h, first by compensating for, then by more than compensating for, the increase in film thickness.