RICERCA ITALIANA     

Innovative techniques for the enhancement of forced convection

Università degli Studi di Udine

Abstract

The research project is concerned with innovative techniques for the enhancement of forced convection in compact heat exchangers and heat dissipating equipment. These techniques encompass the optimal use of passive devices, such as wavy flow passages, vortex generators, corrugated and interrupted fins, pin fins and the realisation of active devices propelled by MEMS (Micro Electro-Mechanical Systems).

In many situations of technical interest, heat convection and mass convection occur simultaneously. Examples are recovery of sensible and latent heat in combustion plants and dryers, and dehumidification processes in air conditioning. An important consequence of water vapour condensation is that total heat conduction loads on the fins increase, bringing about a decrease of fin efficiencies well below dry values (generally close to unity). In such instances, the simplifying assumption of constant temperature boundary conditions on solid walls is no longer valid and the traditional design and optimisation methods, based on the well-known analogy between heat and mass convection, cannot be utilised any more. To fill such a knowledge gap, here we propose a well co-ordinated series of experimental and numerical investigations.

Transient behaviour is another aspect of compact heat exchanger operations that has not been fully investigated yet. Here we propose investigating, both experimentally and numerically, the transient behavior of compact heat >>>

Principal Investigator

Gianni COMINI Università degli Studi di UDINE

Research Objectives

The present research aims to help the Italian industry to stay competitive on the market of compact heat exchangers, both of the plate-and-fin and of the tube-and-fin types. In fact compact heat exchangers are a mature product, and the Italian industry must come out with innovative applications to stay ahead of Asian producers. Users are particularly sensible to the ratio cost/ performances and it is clear that the Italian industry must essentially compete on the performance front. Our research proposal thus includes a series of studies on high efficiency fins, capable of considerably increasing the heat transfer on the air side (the traditional weak spot of compact heat exchangers).

The enhancement of convections allows, above all, to produce more compact and, consequently, less expensive heat exchangers. Furthermore it yields smaller temperature differences in condensers and evaporators and, consequently, makes up for the decrease of performances brought about by the new refrigerating fluids and the materials which are compatible with them. In this way it is possible to occupy the most important market niche where the priority is the substitution of the present refrigerating fluids with the new fluids more environment friendly. Finally, the achievement of high convection coefficients with relatively low air velocities will yield to a considerable reduction of sound emissions with respect to traditional equipment.

It must be pointed out >>>

Timescale

24 months

National and international background

In the past few years, the enhancement of forced convection has been investigated, both experimentally and numerically, in compact heat exchangers, of the tube-fin and the plate-fin types, and in heat-dissipators for the electronic industry. Studies in the past, however, have concerned, essentially, sensible heat transfer in the steady state and the use of passive systems for the enhancement of convection. The most important technological developments, proposed in this research, will deal with the use of
- compact heat exchangers under conditions of simultaneous heat and mass convection,
- compact heat exchangers under time varying conditions and
- active systems for the enhancement of forced convection in heat dissipators for the electronic industry.

The scientific foundation of this studies is given by the very large number of contributions to the technical literature published in the past by researchers from our group, and by the knowledge of the international technical literature acquired by them in the course of their researches. The considerations outlined in the following and the vast bibliography reported at Point 2.2.a support these statements.

COMPACT HEAT EXCHANGERS
Compact heat exchangers are characterised by high surface to volume ratios (of the order, at least, of 700 square meters per cubic meter). The availability of these exchangers has revolutionised traditional heat transfer processes in such fields >>>