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DIPARTIMENTO DI INGEGNERIA INDUSTRIALE
RESEARCH GROUPS
 
Universita di Padova

 

LABORATORY OF HEAT TRANSFER IN MICROGEOMETRIES
Director: Prof. Luisa Rossetto

 

Staff

Ing. Andrea Diani, Post-Doc Research Fellow

@mail: andrea.diani@unipd.it

 

Research Topics and Experimental Facilities

 

1. Heat transfer and fluid flow during single phase forced convection in Micro-Geometries

- Experimental facility

- Air forced convection in metal foams: Experimental Analysis

- Air forced convection in metal foams: Analytical and Numerical Analyses

2. Two phase heat transfer in metal foams

3. Mini Vapour Cycle Sytem (VCS) for electronic thermal management

 

Heat Transfer and Fluid Flow during Single Phase Forced Convection in Micro-Geometries

In the last decades, high heat flux dissipators and compact heat sinks are in more and more demand in many different applications, where an efficient heat transfer would allow the overcoming of the air cooling limits. Because of its poor heat transfer properties air always flows through enhanced surfaces: metallic foams, periodic structured materials, compact heat transfer surfaces, and microgeometries. The aim of this research project is to promote a systematic analysis of new enhanced surfaces for electronic thermal management and air conditioning applications. A new experimental facility has been developed, designed and built in order to run the measurements of the heat transfer coefficient and pressure drop during air forced convection through enhanced surfaces and micro-geometries.

Experimental Facility

This test rig is an open- circuit type wind tunnel with a rectangular cross section and it has been designed and developed to study the heat transfer and the fluid flow of air through several different enhanced surfaces, among which plain and louvered fins, offset strip fins, periodic cellular structure materials, honeycombs, metal foams, and micro- geometrie. The rig is built in stainless steel AISI 316L and it can be subdivided into two main parts: the air compression one, where the ambient air is compressed at a constant gauge pressure of 0.7 MPa, and the test part. The main components of the air compression part are as follows: a screw compressor, a R134a drier, filters, and a 500 l air receiver. The second part of the test rig (see figure top-right corner) consists of: a pressure control valve, a calibrated orifice flowmeter (see figure in bottom-right corner), a 70 L calm chamber, a 1.1 m long connection tube, and a test section where the samples are located. The air passes through the test section where exchanges heat with the enhanced surface that is electrically heated. The air temperature at both inlet and outlet of the test section, the absolute pressure and the pressure variation duirng the air flow are measured. Finally, the air passe through a second valve and it is discharged in the atmosphere.

 

Air Forced Convection through Metal Foams - Experimental Analysis

Metal foams are a class of cellular structured materials that present a stochastic interconnected pores. distribution mostly uni- form in size and shape. In the past decades, these porous media have been largely studied because of their interesting properties; in fact, metal foams are lightweight, while offering high strength, rigidity, and high heat transfer area per unit of volume. Open-cell metal foams present high surface area to volume ratio as well as enhanced flow mixing and attractive stiffness and strength. These properties make them suitable as enhanced surfaces for multifunctional, efficient, lightweight and compact heat exchnagers. The most important geometrical characteristics of the metal foams are the number of pores per inch and the porosity. The metal foams samples tested during the experimental campaingns were made of copper and aluminum with 5, 10, 20, and 40 PPI and high porosity, greater than 90%. The next figures show some examples: an aluminum foam (left) and two different copper foam samples (right).

Heat transfer coefficient as a function of air mass velocity. Data for 20 mm high copper foams.

Pressure gradient as a function of air mass velocity. Data for 20 mm high copper foams.

 

Air Forced Convection through Metal Foams - Analytical and Numerical Analyses

The experimental measurements permitted to develop two models for the evaluation of the heat transfer coefficient and pressure drop during air forced convection in metal foams. These models can be successfully implemented to design optimized heat sink with metal foams as extended surfaces. Recently, a numerical analysis has been started; the direct simulation of the fluid flow through a real foam obtained from micro-tomography scans shows promising results.

From the real foam to the direct numerical simulation of the fluid flow.

 

Two-Phase Heat Transfer in Metal Foams

A new experimental set up was built to study the two-phase heat transfer through metal foams. It consists of three loops: the refrigerant, the cooling water and the hot water loop. The rig was designed for heat transfer and pressure drop measurements and flow visualization during both vaporization and condensation of pure refrigerants and refrigerants mixtures inside structured micro-geometries.

In the first loop the refrigerant is pumped through the circuit by means of a magnetically coupled gear pump, it is vaporized and superheated in a brazed plate heat exchanger fed with the hot water. Superheated vapor then partially condenses in a pre-condenser fed with the cold water to achieve the set quality at the inlet of the test section. The refrigerant enters the test section at a known mass velocity and vapor quality and then it is vaporized by means of a calibrated Ni-Cr wire resistance. The fluid leaves the test section and goes to a post-condenser, a brazed plate heat exchanger, where it is fully condensed and subcooled. The subcooled liquid passes through a drier filter and then is sent back to the boiler by the pump. A damper connected to the compressed air line operates as pressure regulator to control the saturation condition in the refrigerant loop. The following pictures show the test section and a copper foam samples instrumented with 20 T-type thermocouples.

 

Two-Phase Heat Transfer in Microfin Minitubes

The refrigerant charge minimization is one of the most issues for the new environmental challenges. In the last decades, minichannels heat exchangers have been used both in automotive and motorcycle applications as condensers and in air conditioning equipment. Microfin tubes can also be used as air-liquid heat exchangers for heat pumps applications and for refrigeration applications during condensation and vaporization. The possible downsizing of microfin tubes can lead to more compact and more efficient heat exchangers in order to reduce the refrigerant charge.

In the same experimental set up of the two-phase heat transfer in metal foams, microfin minitubes are being tested. These minitubes have an internal diameter of 3 or 4 mm. This experimental campaign is focused on the measurements of the heat transfer coefficients and pressure drops during flow boiling of R134, R1234ze and R1234yf.

 

Phase Change Materials Heat Transfer (Solid - Liquid)

Phase Change Materials (PCMs) are material used as energy storage through their latent heat of fusion. These materials are typically solid at ambient temperature, but when they are heated up above a certain temperature, they melt.

A new experimental facility has been built in order to study this kind of materials as heat sinks for electronic cooling management. Phase Change Materials now under study are paraffins, which have different melting points. The paraffin is located in a bachelite test section over a copper heater. The test section is inserted in a Teflon block to reduce heat losses.

In order to increase the thermal behavior of these paraffins, also paraffins embedded in metal foams are under investigation. 4 different copper foams with different number of pores per linear inch but similar relative density are being investigated.

In the picture on the left, the melting front is showed for a paraffin embedded in a copper foam. Due to the fact that the paraffin is heated from the bottom, the melting front moves from the bottom to the top.

 

Mini Vapour Cycle System (VCS) for Electronic Thermal Management

As the number of electrical and electronic systems in aeronautical applications increases, their physical sizes decrease, and the spacing between electrical components decreases, both the total amount of heat generated (and thus which needs to be dissipated) and the power density (the heat generated per unit volume) increases significantly. In this scenario, the University of Padova operates through a Internal Projects (PRATROSS10) and is one the partners of an European Project PRIMAE (www.primae.org). In this research project, a mini-VCS has been developed and built.

A reference miniature refrigeration system consists of four main components: a compressor, a condenser, a throttling device and an evaporator, also named cold plate. The condenser is split in two water cooled heat exchangers, which condense and subcool the superheated gas. The condensers are two tube-in-tube heat exchangers each fed by two independent water flow rates controlled by two thermostatic baths Then the subcooled liquid reaches the Coriolis effect flowmeter, which measures its mass flow rate with an accuracy of 0.1% of the reading; after that, a needle micro-metering valve is used as the throttling device. The evaporator (see schematic and photo below) is obtained from a 10 x 20 x 400 mm copper plate; three guides have been milled on the top side of the copper plate where a 2 mm ID tube has been brazed for a total length of 1.2 m; on the bottom side, other two guides have been machined to hold a Ni-Cr resistance wire, which simulates the heat load being powered by a stabilized DC power supply. The micro-compressor is a prototype oil-free unit supplied by Embraco, which operates at constant speed; the cooling capacity is controlled by varying the displacement of the compressor.

 

 

The experimental work permitted to obtain the thermal performance of the mini-VCS and of the cold plate. Furthermore, a numerical study of the convection and conduction heat transfer processes inside the cold plate has been carried out. The following pictures show the temperature profiles calculated.