FAN AIR FLOW & ACOUSTIC ANALYSIS
EXTERNAL LAPTOP COOLING FAN
Two quiet fans blow air against the bottom of a laptop to provide additional laptop cooling. This analysis will predict fan volume flow rate, assess fan cooling effectiveness and assess fan assembly radiated noise.
Cooling Fan Assembly
The high speed air flow pattern beneath the computer surface was analyzed using ALGOR, CFD software. Fan blade rotation was modeled at 1000 rpm with unsteady fluid flow analysis.
AIR VOLUME FLOW RATE ANALYSIS
82mm Quiet Fan Design
Velocity Magnitude Analysis
The fan air flow is analyzed between upper and lower surfaces of the fan assembly. The fan was rotated at 1000 rpm and volume flow rate was predicted.
Volume Flow Rate Analysis
Fan volume flow rate is predicted to be 10 cfm at 1000 rpm for this blade design. This blade design is not an accurate match of industry standard fan blade designs, however CFD analysis results are similar to published results for standard 82mm fan designs.
Fan volume flow is significantly impacted by upstream and down stream flow conditions. The quiet fan was placed in a two diameter long tube and the CFD analysis was repeated results revealed that that the volume flow rate at 1000 rpm was reduced from 10 cfm to 6 cfm.
Velocity Magnitude Analysis
Velocity Vector Analysis
QUIET FAN ASSEMBLY FLOW & THERMAL ANALYSIS
Cooling Fan Assembly Velocity Vector Analysis
Results from the unsteady air flow analysis of the assembly was used to perform an uncoupled steady state thermal analysis of the bottom of the laptop. Results revealed that laptop external cooling with these two external fans is not effective at 1000 rpm.
Cooling Fan Assembly Thermal Analysis
QUIET FAN ACOUSTIC ANALYSIS
CFD results also revealed that high speed air flow occurs at the tip of fan blades
Air flow moving over stationary object such as a fan blade, generates dipole aerodynamic noise. This air rush noise is dominated by the relative velocity (u) between the the object and air. The sound power of the interaction is estimated in the following equation:
In this case radiated sound power is approximately 18 dB @ 1000 rpm. Fan radiated sound power is proportional to blade tip speed or fan speed. Lowering radiated air rush sound power while maintaining air flow rate will require an improvement in blade flow propulsion efficiency. A quiet fan will deliver sufficient air at a low rotation speed.
An increase in flow rate from 10 cfm to 16 cfm will require a fan speed increase from 1000 rpm to 1500 rpm. Fan sound power will also increase however from 18 dB to 29 dB. Quiet fans produce high volume flow rates at low rotation speeds.
2-D CENTRIFUGAL FAN BLADE ANALYSIS
Fan noise level reduces with a reduction in fan blade speed however fan air flow rate reduces with blade speed as well. A quiet centrifugal blower will have high air flow rate at low fan blade speeds. Air flow analysis of centrifugal fan blade designs can be performed with two dimensional CFD analysis. In this example a constant thickness fan blade will be compared to an airfoil shaped fan blade. The air foil fan blade improved flow fate by 8% over the constant thickness blade.
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