航空电子设备的强制对流冷却

Forced convection cooling of airborneelectronics

Yannick Assouad,

Thermal and Mechanical Analysis Group Thomson-CSF Radars & Contre-Mesures

Fin pitch = 25.01 per in = 985 per m

Plate spacing, b  = 0.200 in = 5.08 x 10-3 m

Fin Length =  0.111 in = 2.8 x 10-3 m

Flow passage  hydraulic diameter 4rh = 0.004905 ft = 1.50 x 10-3 m

Fin metal  thickness = 0.004 in = 0.102 x 10-3 m

Total heat  transfer area/volume between plates, ?= 719.4 ft2 / ft3 =  2,360 m2 / m3

Fin area / total  area = 0.881

Design methodology

To design an air cooling circuit, theengineer must consider carefully the following parameters.

Electronic circuit board power dissipationsurface density: For power dissipation densities less than 500 W/m2 (0.05W/cm2) on the surface of a fully populated PCB (Printed CircuitBoard), free convection is favored. For power dissipation densities greaterthan 4000 W/m2 (0.4 W/cm2), forced air coolingshould not be used, regardless of the velocity that can be obtained, and moreefficient cooling techniques (heat pipes, liquid cooling, boiling) should beconsidered [1]. This upper limit for forced air cooling is dictated by the factthat heat sinks cannot be used on the circuits due to the lack of space inavionics racks. The boards are designed for a minimum pitch. In between thesetwo limits, forced convection is well suited.

Environment: Although air is readily available onan aircraft, different qualities of air do exist and may determine differences indesigns.

Pressurized bay: this is the most favorable casebecause forced air cooling can be obtained using a fan. The designer can:

calculate a flowrate Qfrom the electronicequipment's overall power dissipation, and air temperature increase through theequipment, using the following formula:

Q = Power/((T_out-T_in)xpxc_p)

where p and cp are the airspecific mass and specific heat.

·calculate the equipment pressure drop△P at flowrate Q:

△P = f(Q)

·select a fan from a fan manufacturer's catalogue usingthese two characteristics (Q, △ P).

It is important to check that the airvelocity obtained on each circuit provides a convective heat transfercoefficient sufficient to limit the temperature gradient between the componentsand the air. If not, it will be necessary to check the uniformity of air flowdistribution on each board. Local heat sinks can also be used on high powercomponents, if space is available. Alternatively, a more powerful fan can beselected.

Depressurized bay: the decrease of air density when thealtitude increases must be taken into account when choosing the fan. It isimportant to check that the component temperatures will remain within theprescribed limits.

Defining, at atmospheric pressure,

Qv0: fan volume flowrate

Qm0: fan mass flowrate

P0: pressure delivered by the fan

△P0: pressure drop of theelectronic equipment at Qv0,

P0: air density;

and at altitude z which corresponds to a static pressure Pstat,

Qv: fan volume flowrate,

Qm: fan mass flowrate,

P: pressure delivered by the fan,

△P: pressure drop of the electronicequipment at Qv,

P: air density, then the followingrelations apply:

P/P₀=P_stat(bar)

Q_m/Q_m0= P/P₀

which means that the mass flowrate willdecrease at the same rate as air density, which in turn will yield a greaterair temperature increase going through the electronic equipment. This may notbe compensated by the static air temperature decrease which occurs withaltitude.

We also have:

P/ P₀ = P/P₀

△P/ △P₀ = P/P₀

which means that the pressure delivered bythe fan will decrease but will be compensated by the decrease of pressure lossat Qv. The motor glide may change with air density, which will havean impact on the fan flowrate and delivered pressure and thus sh should not beneglected, especially if the altitude range is wide.

Then the design steps are the same asthose applied to the previous case.

Aircraft Environment Control System (ECS)air: aboard anaircraft, especially fighter aircraft, there exists an ECS which can providecool air to electronic equipment. Flowrate and pressure can generally beprovided in sufficient quantities to cool electronics efficiently.

This type of forced convection coolingcreates problems due to air pollution. This air contains particles such asdust, sand, metals and fluid droplets, typically oil, kerosene and liquidwater.

It is therefore unwise to pass this airdirectly over the electronic components due to the risk of decreasedreliability from micro short-circuits, abrasion or chemical degradation of thecomponents.

Electronic equipment should be designed toavoid contact between the components and the pollutants: air is passed throughcold plates installed either directly at the board level (see Figure 1) or atthe rack level (see Figures 2 and 3).

Figure 1 shows a compact finned cold plateembedded in a double sided circuit. This is an Air Flow Through module (AFT).The heat generated by the electronic components is transferred first byconduction through the board and then by forced air convection. The forcedconvection heat transfer coefficient is enhanced by the compact fins of thecold plate. Air is usually directed along the width of the circuit boards inorder to limit the pressure drop.

Figure 2 shows a rack which has two coldplates cooled by ECS air. The rack includes circuit boards which are cooled byconduction enhanced by a metal (generally aluminum or copper) plate in contactwith the wall cold plates through thermal clamps located on each side of thecircuit as shown on Figure 3.

Design tools

Two kinds of design tools are required forthe analysis of the forced convection cooling methods described in the previousparagraphs:

a CFD code to calculate pressure drops, air PCBs or infinned cold plates,

a thermal analysis code to check that component junctionand/or case temperatures are within the prescribed limits when cooledcoefficient given by the CFD code.

Iterations are generally necessary betweenthe two types of calculations to converge to a satisfactory solution.

CFD calculations: Figure 4 shows an example of aFlotherm CFD calculation for six fan cooled PCBs. In this case, the CFDcalculation has three purposes:

to aid in fan selection, calculating the pressure dropfor the prescribed flowrate;

to design the air distribution and collecting chambers,so that the air velocity is uniform for each PCB, assuming approximatelyuniform power dissipation. Figure 5 shows the upper view of the rack and theair distribution among the five PCB's;

to get air velocity and air temperature on each PCB,which will constitute the boundary conditions for the thermal analysis of eachcircuit, Figure 6 shows the air velocity distribution on one of the circuitboards.

Thermal analysis: Once the boundary conditions have beenobtained from the CFD calculation, a thermal analysis may be performed on any partof the circuit. However, two other essential points must be considered:

the convective heat transfer coefficient on the componentfaces and on the PCB must be evaluated. Fully populated circuits are verycomplex objects from a fluid dynamics point of view, which explains whyliterature correlations, classically used to estimate convective heat transfercoefficients fail to give correct values for electronic cooling. The heattransfer coefficient is usually underestimated using these of correlations or CFDcalculations; Thomson-CSF Radars & Contre-Mesures has developed its owncorrelations from experiments, over a ten year span.

Compact thermal models for each component populating thePCB are required because, detailed models of each component would take hours ordays to run on a computer; the European project DELPHI methodology and results[2] have helped considerably to increase Thomson-CSF and other partners abilityto create reliable compact models.

Figure 7 gives an example of the thermalanalysis performed on one of the circuits of the previous rack, using a boardlevel thermal analysis tool, VTAT (Visula Thermal Analysis Tool) of ZukenRedac.

Cooling method summarized

Based on previous work conducted withinThomson-CSF - including both CFD calculations and thermal analysis checked withexperiments - a ranking of the three different types of forced convectiondescribed at the beginning i.e. direct air forced convection, AFT andconduction cooled modules in an air cooled rack, can be created.

The ranking is as follows:

1. Direct air forced convection oncomponents may authorize power dissipation densities on a fully populatedboard, up to 3000 W/m2, with an air velocity in the range of 5 m/s.This type of design requires non-polluted air and a powerful fan on account ofthe high velocity of 5 m/s which will generate high pressure drops.

2. An AFT may allow power densities up to3000 W/m2 or more, but with a very large pressure drop penalty.For 3000 W/m2, the pressure drop will be in the order of 5 mbar fora 100 mm wide module which yields, for a 200 mm wide classical module, a totalpressure drop of 10 mbar, not accounting for the distribution and collectingair chambers [1].

3. Conduction cooled modules in an aircooled rack are limited to power densities up to 1500 W/m2, due tothe parabolic temperature gradient from conduction in the metal plate embeddedin the PCB (see Figure 8).

Conclusion

Thermal control of airborne electronicshas allowed the development of the advanced air forced convection coolingtechniques which have been presented here. Furthermore, increases in powerdissipation densities combined with the constraint of lightness will requirethe investigation of even more efficient cooling techniques such as liquid flowthrough modules, flat heat pipes and direct evaporation. The two latter techniquesstill require improvements before use on board aircraft on account of theirsensitivity to stressed mechanical environment.