How we can improve Active Cooling System? We were be able to reduce in our cooling system the power consuming of FAN and Thermal Control Circuitry (TCC) up to (1.5 – 3.0)% of total input power of LED fixture. Itwas result of the implementation of the concept of US patent 8070324.
The increasing of efficiency of active cooling can be reached by two ways at least:
- firstly – by reducing of electrical power for air activating (electrical side);
- secondary – by increasing of convective heat transfer (physical side).
However the optimization of thermal physic processes in the construction of LED fixture (first of all in Heat Sink) is very significant. Actually this is the subject of this review.
The typical construction of traditional Heat Sink (HS) is not very suitable for LED fixture even in combination with FAN (cooling module, or Fan Sink). Because of solidentirely of the base of HS the forced air from FAN can not activate the area directly nearthe hottest spot of LED fixture (COB or LED array itself). Thus the heat transfer isrealizing in two steps in principal: by thermal conductivity from LEDs (as heat source) through metal body of PCB and HS consequently to pins (or fins) of HS, then bythermal convection process the heat is dissipating from surface of pins into nearest layerof air surrounding of HS.
It is known that thermal convection can be determined by Newton law in the formula:
Q- power of thermal flow, [Q] = [W] ;
- S - area of dissipating heat surface, [S] = [m^2]
- T*h – temperature of dissipating surface (on the edge of pin of HS), [T] = [*C]
- T*a – ambient temperature (surrounding HS air), [T] = [*C]
- @ - ‘alpha’, coefficient of convection heat transfer, [@] = [W] / [m^2] [*C]
Analysis of (1) shows clearly the motivation of designer of HS to increase Sparameter for increasing of capability of heat dissipation. However the increasing of S-parameter as usual is limited by such requirements to LED fixture as reasonable size and weight of HS.
Moreover some pitfall exist here in the design of HS, for example: the increasing ofS parameter (with more number of pins) can caused the decreasing of @ parameterbecause of difficulties with moving the air (even forced by FAN) between pins. Thevalue of @ which could be realized in practical design is very significant and sensitive parameter for capability of cooling system (both, passive and active).
Evidently, active cooling capable to provide more higher value of @ (roughly up to100 W/m^2*C instead of 8-12 W/m^2*C for passive). That’s why the active cooling has advantage in efficiency in principle to compare with passive HS.
However the physical nature of @ parameter is very complicated and dependsstrongly on many construction factors of HS (size and shape of pins, for example) and conditions of heat exchange in the boundary layer of air around of the pin (viscosity of medium, temperature gradient, etc). Fortunately even under this complicated conditions we have some possibilities for increasing the @ value without increasing the electrical power of the FAN (evident, but less effective way).
One of productive idea is generating the turbulence around source of heat (vortex effect). Vortex is very valuable effect because it increases the @ parameter without any additional energy consumed by cooling system, just by right design (in terms of size and shape mostly) the elements of physical channeling forced air through body of HS, PC Band other constructive elements of LED fixture.
We need to underline that this way of design is empirical significantly because of complicating in the getting of math model for this thermal physic processes in cooling system. One of the practical version of vortex generator can be illustrated by Fig. 1
In this construction the vortex is forming by system of openings (holes) in thebody of HS and PCB which caused the turbulence inside of each physical channel formoving air by means of cavities of various shapes and sizes. The output of each channelshould be located close to heat source (COB or LED array) as much as possible. In thisway the convection heat transfer can be applied directly to the hottest area of LED fixture. As result the cooling efficiency can be increased even to compare with active cooling on the base of traditional HS design.
The standard HS (from Hong Kong) has been modified for prototype of Demo Kitof Active Cooling Module (ACM 50W) based on the concept of Vortex Generator (see Fig. 2). It has two coaxial rings of opening (holes) which are located around of the central area for heat source – COB LED package 28x28 mm and between fins of HS underneath of the base of HS.
The main constructive parameters of HS are:
- weight –162 g,
- dimensions – dia. 91x40 mm,
- number of fins – 94,
- area of heat dissipating – 746 cm^2,
- number of holes dia. 3 mm with special shape – 28.
Accordingly the PCB (see Fig.3) has similar system of holes and cavities for Vortex generating when assembled by “sandwich” scheme with HS. It has also nest for COB package in the centre and place for Thermo Control Circuitry (TCC) close to the hottest spot of PCB.
These samples for Active Cooling (by Fan) heat sinks illustrate clearly the concept of vortex for increasing of convectional component of heat transfer from heat source - powerful LED fixture. The openings and edges of different shapes, sizes and layout provide the forming of turbulence of air around of surfaces of metal body of heat sink. Hence the coefficient of convection is being intensified at the same dissipated area of heat sink and power of the Fan. This concept has been protected in the claims of US Patent Family, granted to MP Lighting
It's evident that innovative concept of Active Cooling System can not be realized on the base of traditional heat sinks from the shelves of main manufacturers. The requirements to shapes, sizes and specific fitting to modern LED fixtures (in accordance to art design particularly ) dictate the original design of heat sinks and prototyping. It was done by CNC milling directly at R&D Lab on the base of "vhf" equipment (Germany). The efficiency of design, prototyping and testing of new technology was increased dramatically.
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