RICERCA ITALIANA     

Innovative techniques for the enhancement of forced convection

Università degli Studi di Parma

Abstract

Although convective heat transfer fundamentals are well known as a result of the work done by many researchers in the last century, some applications in many both civil and industrial fields still require research work. Heat transfer applications which include convection enhancement techniques certainly deserve further research efforts.

The here presented two year research project is focused on this argument and aims to coordinate the efforts of a part of the scientific national community within the research in heat transfer towards the deepening of both theoretical and applicative knowledge on the mechanisms responsible for the intensification effects in convection heat and mass transfer.

Although the scientific literature on both passive and active heat transfer enhancement techniques is wide and variegated and although it is well-known the important benefits, in terms of cost and size reduction and of energy saving correlated to their usage, the application of these technologies in some industrial fields is affected by a certain delay, or in any case requires the development of some optimization procedures. These difficulties are in part related to the gap, which the researchers involved in the present project hope indirectly to fill, persisting in Italy between the industrial and the academic research fields.

The researches suggested by the five local units are integrated within the global project as complementary activities. They aim to fill some of the scientific and technological gaps, already kwon to the researchers involved into the project, in the field of the research applied to the optimization of heat transfer apparata and to develop, with respect to the current state of the art in the field, significant progress having industrial applicability.

In the first year the project is intended to hone both experimental and numerical methodologies for the modelling and understanding of the phenomena correlated to the intensification of convection mass and energy transfer, while in the second year, the project will be more concerned on the industrial applications.

The research will be focused on the validation of both experimental and numerical techniques with the particular aim of investigating the more critical aspects of the problem, correlated to the three dimensional character and instability of the flow and to the estimation of the heat transfer coefficient, and therefore to the evaluation of the performance of heat transfer surfaces, under dry and wet conditions.

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 behaviour of compact heat exchangers under a large variety of operating conditions, including simultaneous heat and mass convection, evaporation and condensation.

The project is particularly addressed to the heat exchanger industry, with particular reference to the refrigeration and air conditioning field. In this fields, the design of heat transfer equipment is often made critical by the lack of reliable tools for the estimation the heat exchanger efficiency and for the estimation of the benefits brought by a given convection enhancement technique. To fill such a knowledge gap, here we propose a co-ordinated series of experimental and numerical investigations.

The enhancement of forced convection is of paramount importance also in the design of the heat-dissipators utilised by electronic industries and consumer industries. In particular, the continuous increase of circuit density in microprocessors has stimulated the interest for more efficient heat dissipators relying on a new generation of liquid cooled plates with pin-fins of small aspect ratio. To promote turbulence and to improve thermal performances of liquid cooled plates, without increasing the liquid flow rates, here we propose the use of active systems which employ vibrating surfaces, propelled by MEMS.

Also regarding the electric heaters for the consumer industry, thee are surprisingly, open problems. The banks of open-coil resistance wires of the type utilized in these appliances have never been extensively investigated despite their widespread use. This informative gap deserves certainly to be filled. <<<

Principal Investigator

Sara Rainieri Università degli Studi di PARMA

Research Objectives

The here proposed research aims to the deepening of both theoretical and applicative knowledge on the techniques for the intensification of forced convection.
The proposers intend in this way to provide useful tools to help the Italian industry to stay competitive on the market of heat exchangers, in particular of both plate-and-fin and tube-and-fin compact heat exchangers.
In fact, all the five research units have directly or indirectly some connections with manufacturers involved in this field, therefore the here proposed project will have a great effect on the national industry in relation to the ability of designing and innovating the heat transfer equipments.
Compact heat exchangers are a mature product, and the Italian industry must come out with innovative applications to stay ahead of foreign producers. Users are particularly sensible to the ratio cost/ performances and it is clear that the Italian industry must essentially compete on the performances front. Our research proposal thus include a series of studies on high efficiency fins, capable of considerably increasing the heat transfer on the gas side (the traditional weak spot of compact heat exchangers).
The enhancement of convection allows, above all, to produce more compact and, consequently, less expensive heat exchangers.
Furthermore, the development of convection enhancement techniques is nowadays a even more strict demand in the field of refrigeration and air-conditioning field, in order to compensate the decrease of performances brought about by the new environment friendly refrigerating fluids and by the materials which are compatible with them.
Finally, the achievement of high convection coefficients with relatively low air velocities will yield to a considerable reduction of sound emissions and dissipative effects with respect to traditional equipments.

The objectives of the present research have to be framed in this context and will be alltogether below described. The specific objectives of each local research units will be further discussed in detail in section 14.

One of the most critical point in heat exchanger design is due to the onset of convection under “wet conditions”, that is when on the heat transfer surface the vapour condensation occurs.
In this situation, which is frequently encountered in the refrigeration and air-conditioning field, that is in cooling applications, the enhanced fins may loose their effectiveness, since the additional latent heat load and since the presence of a liquid film which decreases the heat transfer.

On the other hand, a careful prediction of the convection enhancement effects may be particularly critical in the conditions above described, that is when convective transport of both heat and mass occurs at the interface, as, for instance, is the case of wet air drying by surface water vapour condensation. While in single-phase heat transfer many, both theoretical and experimental, methodologies aimed to assess the performance of a given heat transfer enhancement technique have been finely honed and validated, the procedures which can assist the design engineer in the choice of an augmentation technique in two-phase heat transfer are less established.

Therefore, the research team intend to study both the internal and external forced convection phenomena also in presence of surface condensation and to hone investigation tools for the assessment of the performance of enhanced heat transfer surfaces. A specific objective of the research is the validation of estimation techniques of the local heat transfer coefficient on wet surfaces. This results is of fundamental importance for the development of optimization procedures in the design of the fins to be used in heat exchangers under conditions of both sensible and latent heat transfer.

Finally we will investigate also the optimization of the thermal behaviour of compact heat exchangers during start up and turning off periods, to supply industries with design methodologies suitable for all the operating stages.

Another field to which the present project intend to give its contribution is related to the development of enhancement techniques based on the destabilization of the velocity boundary layer and then to the encouragement of the onset of a turbulent transfer regime.
These systems are primarly based on both passive (fin-surfaces, such as offset strip, louvered-, perforated- and wavy-fins) and on active (vibrating surfaces) enhancement techniques.

Regarding passive systems, the present project intend to hone the numerical techniques for the modelling of turbulence, by making them applicable also to complex geometries.
Regarding active systems, the project intend to test innovative devices based on surface vibration by means of MEMS (miniaturized piezoelectric actuators), for the convection enhancement also in confined flow condition.

Finally it must be pointed out that we will employ advanced experimental and numerical techniques. As a consequence, new experimental skills will be acquired in the fields of infrared and holographic thermography, speckle photography and laser velocimetry. Similarly, we will contribute to the advancement of computational thermal-fluid dynamics through the production of new codes, and the accumulation of experiences on the use of commercial codes. <<<

First Results

The primary results expected by the present research are analitically described in relation to the role of each research units in section 14.
On the whole the here presented research project aims to coordinate the efforts of a part of the scientific national community within the research in heat transfer towards the deepening of both theoretical and applicative knowledge on the mechanisms responsible for the intensification effects in convection heat and mass transfer.
The primary objective is to fill some of the scientific knowldedge gaps and to deepen still open problems in the field of techniques aimed to the intensification, both by means of passive and active systems, of convection transfer.
The project, scheduled into two yearly stages, aims to bring methodological and technological innovations into the industrial field relevant for the present project.
One of the expected result is the honing of both experimental and numerical methodologies, aimed to the modelling and comprehension of the physical phenomena related to the use of convection ehnancement techniques, both in mono-phase and in surface condensation conditions.
The research team intends also to apply the investigation methodology to situations of industrial relevance.
The expected results will provide useful tools to the Italian industry in order to help it to stay competitive on the market of heat exchangers, in particular of compact heat exchangers both of the plate-and-fin and of the tube-and-fin types.
In fact, the five research units have directly or indirectly connections with industries related to heat transfer apparata, therefore the research project will have a strong effects on the national industry in relation to the ability of designing innovative equipment and devices.
In some industrial fields, the use of convection enhancement techniques is affected by a certain delay or in any case requires the development of some optimization procedures.
These difficulties are in part related to the gap, which the researchers involved in the present project hope indirectly to fill, persisting in Italy between the industrial and the academic research fields.

The researches suggested by the local units are integrated within the global project as complementary activities. They aim to fill some of the scientific and technological gaps, already kwon to the researchers involved into the project, in the field of the optimization of heat transfer apparata and to develop, with respect to the current state of the art in the field, significant progress having industrial applicablilty.

More in detail, the remarkable applicative potentiality are related to:
-The development of experimental methodologies, based on non conventional measurement techniques, aimed to the estimation of the performances of enhanced surfaces also in condition of vapour surface condensation.
-The evaluation of the efficiency of enhancement tecniques based on wall corrugation and to surface treatmens aimed to the encouragement of the dropwise condensation modality.
-The design of heat exchangers employed in the refrigeration and air- conditioning applications. In fact these, like other applicative fields where the heat exchanger operates at temperatures lower than the dew point, require the development of innovative and reliabel procedures to be validated trhough the comparison with experimental data.
-Heat transfer correlation for banks of open-coil resistance wires employed in electric air heaters.
-The formulation of innovative numerical model of the thermal behaviour of microscale heat dissipators.
-The validation and application of active enhancement systems to the convection from finned surfaces by means of mechanical vibrations, actuated by MEMS devices, in confined flow conditions.
-The definition of suitable tools for the thermal control of heat exchangers in unsteady working conditions also in presence of surface condensation.
-The development of codes, based on genetic algoritms, aimed to the estimation of the dynamic behaviour of heat transfer apparata.
-A wide experimental campaign aimed to the collection of a data base regarding the forced air convection on wavy and ribbed channels with ribs variously arranged with respect to the streamwise direction and the development of the corresponding heat transfer correlations.
-The formualtion of turbulence models suitable for complex geometries, like enhanced surfaces.

Finally, by means of reasearch grants to be addressed to persons who will be recruited by the research units, the project intends to obtain the results of training young researchers in a strategic field for the advancement of the italian industry.

The results of the research will be made open by means of communications within national and international conferences in the field of heat transfer and by means of publications on scientific journals.

In relation to the applicative character of the project, the research activity migth provide the oppurtunity also of registering patents of industrial interest. <<<

Timescale

24 months

National and international background

An examination of both the national and international scientific literature in the field of heat transfer, reveals the relevance and novelty of the argument of both passive and active techniques aimed to the intensification of the convection energy and mass transfer.
Studies in the past, however, have concerned, essentially, heat transfer in the steady state and the use of passive traditional systems for the enhancement of convection, primarily to be applied to mono-phase conditions. The most important and innovative technological developments, proposed in this research, will deal with:
-both experimental and numerical techniques aimed to the assessment of the performance of enhanced heat transfer surfaces in simultaneous forced convection transport of energy and mass, also by considering the unsteady operating regime;
-study of heat dissipators which adopt both passive and active systems for the intensification of the convection mechanism.

The scientific foundation of this studies is given by the very large number of contributions to the technical literature published in the past, also 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 below and included in each local project support these statements.

Passive convection enhancement techniques are based on the fluid flow alteration which may be caused by a suitable wall shape. In the laminar flow regime, which frequently occurs in compact heat exchangers, that is characterised by high surface to volume ratios (of the order, at least, of 700 square meters per cubic meter), the required enhancement effect can be obtained by inducing flow instability, whereas for higher Reynolds number values some heat transfer enhancement follows to the turbulence intensification caused by wall geometry.
The traditional way out is the increase of the exchange surface through the use of fins incorporating convection enhancing devices in the form of slits, corrugations and winglets.
The availability of these exchangers has revolutionised traditional heat transfer processes in such fields as heat recovery and air conditioning.
The reduction of flow sections (brought about by the increase of the surface to volume ratio) and the simultaneous reduction of flow-through velocities (brought about by the necessity of containing pressure losses and noise emissions) have required a complete revision of traditional design procedures. Although several research findings in this field have already been incorporated into industrial best practices, it cannot be claimed that there is no room left for further innovations.

In particular, operations of compact heat exchangers at surface temperatures below the dew point are not fully understood yet. For example the well-known analogy between heat and mass convection is not sufficient for the design of compact heat exchangers operating under conditions of intensive dehumidification. When intense condensation of water vapour takes place on fin surfaces, latent heat effects greatly increase the total heat conduction load on the fins. Consequently, fin efficiency decreases well below the corresponding dry values (generally close to unity), and the simplifying assumption of constant temperature boundary condition on solid walls is no longer valid.
The careful prediction of the convection enhancement effects may be particularly critical in these conditions.
Under wet conditions, the majority of the heat transfer enhancement techniques are intended to promote the onset of the dropwise condensation modality and to encourage the condensate drainage. In fact, it is well-known that the sweeping and renewal of the droplets growth process is responsible for the high heat transfer coefficients associated with dropwise condensation, as compared to the film condensation condition. This goal can be obtained by applying hydrophobic layers of organic substances, inorganic compounds, polymers or noble metal coatings which reduce the wettability of the surface.
When surface condensation occurs, the theoretical approach to the evaluation of the effect of the heat transfer enhancement technique becomes practically unworkable, because of the difficulty of accurately modelling the heat and mass transfer mechanism at the wall to fluid interface, due to the presence of the hydrophobic coating.
An expeimental estimate of the local heat flux at the wall to fluid interface may be obtained from the solution of an inverse heat conduction problem, in which the wall temperature is known, while the interface heat flux is the unknown quantity. However, this inverse problem falls within the ill posed ones, for which small measurement errors make the measured temperature useless to restore the interface heat flux.

Also regarding the application of the numerical analysis to the evaluation of the benefits attainable by a given convection enhancement technique there are still open problems which require further research efforts. For industrial purposes, turbulent models , natively implemented in the commercial code such as those of the k-epsilon family, have been widely used thanks to their stability and lightness. However they cannot reliably reproduce all kind of phenomenon occurring in complex flow situations. Therefore the development of higher order non-linear turbulence models, also called NLEVM (Non Linear Eddy Visocity Model), or other models based on the LES (Large Eddy Simulation) aproach are essential in order to extend the use of CFD (Computational Fluid Dynamics) tools to the analysis of complex geometries having characteristic elements with sizes close to those of the structures to be excited.

Moreover it must be noted that in most of the works available in literature on the topic of this research, the simplifying steady state condition is generally assumed. This hypothesis does not allow to model the heat transfer apparata behaviour and to optimise the related control systems in real operating conditions. One of the sub-argument of the present research sets in this context. It is focused on the investigation, both experimentally and numerically, of the transient response of compact heat exchangers under a large variety of operating conditions, including simultaneous heat and mass convection on the air side, and evaporation and condensation inside the tubes.

The most recent technological developments concerning the enhancement of heat transfer in heat dissipating equipments for the electronic industry are connected with the use of liquid cooled plates equipped with pin fins. Several pin geometries and arrangements have been investigated, and the good performances obtained provide the motivation for continuing the studies in this field, with the aim of further increasing the heat flow rates per unit surface that can be dissipated by liquid-cooled plates. To this purpose, the use of active convection enhancing devices has been successfully tested. These devices can promote turbulence through the vibrations induced on the plate surface by MEMS.

Regarding the application to electric air heaters, helical coiled resistance wires are the most cost-effective solution for many home and industrial appliances that utilize electric air heaters. Despite the widespread use of these components, however, forced convection heat transfer from helical coiled resistance wires, of the type utilized in many household applicances, has not been much considered in the literature. The feeling of a part of the research team is that numerical simulations can fill this informative gap and lead to heat transfer correlations that can be utilized for design purposes.

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