Heat Sink

 

 

HEAT SINK TECHNOLOGY

 

Integrated Augmented Fan/Heat Sink Overview

FIGURE 1 – Thermal Performance vs Volume

Figure 1 fan heat sink thermal values as reported by heat sink vendors were plotted as a function of total volume. The wide range of values observed for a given volume is primarily due to variations in the cfm output of the fan. There are generally high and low practical limits of thermal performance based upon the size of the fan used to impinge air on the heat sink. The LAC Integrated Augmented Fan breaks through these lower limits as shown by the graph above – 1/2 to 1/3 the thermal resistance.

 

FIGURE 2 – Prototype 1 copper heat sink assembly.

 

Figure 2’s Preliminary Prototype 1 was made out of copper to minimize performance degradation due to base spreading resistance. Overall volumetric dimensions are 2.5″ x 3.15″x 1.00″
Figure 3 – IR image of Prototype 1 set at angle view.
 
Figure 3’s infrared imaging was used as a non intrusive measurement technique so as not to obstruct air flow patterns due to thermocouple wires. In addition, the infrared camera could observe the entire heat sink assembly and adjusted to highlight small changes in temperature. The infrared imaging equipment used was a Cincinnati Electronic Avio model TVS 2300 MKII ST instrument. The IR images were taken immediately after the data in table one was collected. Temperature and emissivity compensation of the IR instrument was compensated for by calibrating the color coded temperature readout of the IR image to the temperature reading from a hardwired thermocouple that probed the surface of the copper heat sink assembly. The purpose was was to examine the interaction of the air flow with the fin/ring, the sensitivity of the IR equipment was adjusted to highlight the magnitude and distribution of the temperature gradients on the ring plates. The image shown in Figure is a composite temperature average captured at 32 frames/second.
Figure 4 – IR Image Over Laid on Heat Sink – From Angle View

 

Figure 4 is an overlay of Figure 3 with a photo of the test heat sink to more easily see the relative temperature distribution. It can be observed from Figures 3 & 4, an annular region of cooler temperatures on the surface of the rings nearest to the fan blades. The surface temperature in this enhanced annular region is approximately 25 deg. C and has a temperature gradient which is predominately radial in nature. This indicates heat transfer dominated by the influence of the fan. These cooled regions penetrate the apertures to the extent of about 1/2 the diameter of the fan this penetration is about 25% of the propeller radius R or main air flow passage radius R. The following graph (Figure 5 below) helps to demonstrate the magnitude of cooling characteristics showing the temperature gradients on the fin/rings.
Figure 5 – IR Imaging Analysis – Angular View

 

In the horizontal cross section depicted in Figure 5 above, a parabolic curve, with a flattened bottom (indicating a uniform temperature distribution), is observed in the region between pixels 60-115 and 160-195. It is observed from the profiled image that a strong influence originating from the tip vortex impacting on the adjacent internal ring structure results in a relatively high heat transfer coefficient. The narrow band of effectiveness in this region may be explained through examining the structure of the shed vortex. Cohesive forces, such as internal velocity and pressure, in the central region of the vortex, causes the behavior of the vortex to rotate as a rigid body. This rigid body may be modeled as a disc where the velocity is directly proportional to the radius. Consequently, the fastest velocity is found around the circumference of the vortex or the origin of a region where a high heat transfer coefficient is developed. As the core of the vortex moves further away from the source, the cohesive forces break down, resulting in a breakdown of rotational flow. The edge of this core becomes a free vortex or a more viscous fluid and its velocity (energy) changes as a function that is inversely proportional to the radius. The result is lower velocity and heat transfer coefficients resulting in rising temperatures.

How It Works:
The Integrated Augmented Fan/Heat Sink utilizes the normally wasted energy in the tip vortex of the fan in a dual fashion. First the mass airflow of the fan system is increased by approximately 30% – the penetration of the tip vortex between the rings creates a negative pressure field which induces a secondary flow, adding to the overall fan flow. Secondly, the penetration of the tip vortex between the rings and on the inner diameter creates a high velocity scrubbing action for a high heat transfer coefficient.

Noise Reduction:
Fan noise is reduced 3 – 5dBA through the destruction of the the tip vortex, the secondary air flow enters the fan counter to the rotation of the tip vortex impeding it’s formation and travels down the negative pressure side of the fan blade to establish a smoother, more uniform flow distribution. As this was not the focus of our development on the Initial Prototypes further gains here have yet to be realized as evidenced by Production fans manufactured by NMB and Panasonic, which average on the order of an -8dBA reduction. Additional reductions can be realized by reducing fan RPM so that the Augmented Fan heat sink operates at an equivalent thermal resistance to current market performance requirements. This would, in most cases, reduce sound levels well below the threshold of hearing.

Bearing Life:
Reducing Fan RPM as indicated above would increase bearing life 300% (bearing analysis available on request).

Air Flow Pattern:
Air flow patterns of the integrated augmented fan/ heat sink were qualitatively studied by using flags made of a lightweight tissue. These flags were brought into close proximity to the various apertures of the heat sink to determine the air flow pattern. The results are shown in Figure 6 below.

 

Figure 6 – Qualitative Air Flow Pattern

In Figure 7 below, an interpretive air flow cross sections of the pattern shown in Figure 6 is generated to visualize the probable air flow scenario.

Figure 7 – Heat Sink Cross Section Showing Air Flow Pattern.
Test Methodology:
The heat sink was powered to 30 watts by applying 10 watts to 3 heating elements. The bottom of the unit was placed on a thermal insulating foam to minimize losses through conduction to a cold body. The unit was allowed to stabilize for two hours.; The fan RPM speed was varied by adjusting the input voltage to the fan. The RPM speed was determined by strobe scope (Edmont Scientific P/N E 771500). Heat sink and ambient temperature measurements were monitored using a 40 AWG, copper constantine thermocouple and recorded on a Mokogawa Hokushin Electric Model 3081 Data Recorder
 
Figure 8 – Prototype II Aluminum 2.5″ x 2.0″ x 1.0″ (Preliminary Production Prototype)
Compatible to High Volume Manufacturing Processes

  • Minimized Secondary Machining Operations
  • Minimum Volume
    Accordingly, a heat sink with a total footprint of 2.5″ x 2.0″ x 1.0″ was designed to be manufactured using an extrusion process as pictured in Figure 8 and results noted in Figure 1.Conclusions:
    As shown here in our test results overview, the LAC Integrated Augmented Fan/Heat Sink performance was increased 2X over current industry performance ranges of operation. With further refinement of fan and heat sink, design and integration projections for a 3X improvement is readily obtainable with the current available knowledge base. In addition, extensions of bearing life and significant reductions in noise are positive features that can be incorporated in a final manufactured design.