Air Flow BenchPresented By: Saket Karajgikar & Nikhil Lakhkar

Advisor: Prof. Dereje Agonafer

Air Flow Experimental BenchReference: www.fantester.com

Air flow bench ConfigurationReference: www.fantester.com

Experimental Bench ContdThe chambers are designed in accordance with AMCA 210-99/ASHRAE 51-1999 and have been sized for convenient flow rangesThe chamber diameter is determined by the size of the axial flow fan to be tested and the maximum flow range desiredLower flow ranges may be achieved by utilizing smaller nozzles in the nozzle array

Experimental Bench ContdThey are positioned on the plate so that they may be used in parallel to achieve higher flow ranges.Stoppers are provided to block off nozzles not in use and are easily removed for different ranges of testing.

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Experimental Bench ContdThe chamber has flow straightening screens installed upstream and downstream of the nozzle array. The screens break up turbulence in the air stream and provide a uniform flow approaching the nozzle array.

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Experimental Bench ContdThe flow through the chamber is controlled with a sliding gate valve called a blast gate. By opening the blast gate, the flow is varied through the chamber to provide test data from shut off (no flow) to free delivery (no back pressure) for fan performance evaluation.

Reference: www.fantester.com

Applications of Air Flow BenchAir Flow Bench is used for:To calculate the Air Flow RateFan Performance Curve MeasurementThermal Resistance

Air Flow Rate

where, Q = Air Flow Rate (m3/min) A = Nozzle Sectional Area (m2) V = Average Flow Velocity through nozzle (m2/sec)

where, g = gravitational acceleration 9.8 m/s2 Pn = Differential Pressure r = Specific Gravity of Air (1.2 kg/M3 at 20oC, 1atm)Q = 60 x A x VV= ( 2 g Pn / r)1/2

Fan Performance CurveA fan performance curve characterizes the ability of the fan to drive air against a flow resistance It is plotted as static pressure drop in inches of water gauge pressure (iwg) against air flow in cubic feet per minute (cfm)The measurement starts with the air flow chamber blocked so no flow occurs (i.e. 0 cfm) and proceeds with greater and greater flow rates until the static pressure has dropped to zero representing the "free delivery" condition

Fan Performance TestingThe purpose of this test is to determine the aerodynamic characteristics of the fan under testData is taken from no flow (shut off) to free flow (free delivery)Curve is plot using these data points

Fan Performance TestingExperimental Set-upNozzle is selected based on required flow rangeNozzles should always point downstreamFan to be tested is mounted on the front plate of the chamberFan should be sealed adequately to prevent leakage

Fan Performance TestingExperimental ProcedureFirst data point is considered at no flow or shut off conditionAt this point differential pressure is zeroStart the counter blower at low speedSlowly open the blast gate until 0.1 inches w.g. is measured for the differential pressureAllow the fan to stabilize and record the data

Fan Performance TestingExperimental Procedure (Contd)Record the data points for different Blast gate openingAs the experiment proceeds, differential pressure increases and static pressure decreasesContinue taking data points till free delivery is reached (I.e zero static pressure)Shut off the counter blower and plot the dataData points fully define the fan performance curve

Typical Performance Curve

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System Impedance TestingPurpose for this test is to determine the pressure required to move the appropriate amount of volume flow through the systemFor the impedance test, the air is forced through the unit to be tested and the pressure drops are measured for various flow points

System Impedance TestingExperimental ProcedureOpen the blast gate completelyStart the counter blower and blow air through the unit to be testedThe first data point should be a minimum of 0.1 inches w.g. differential pressureTake 5 to 6 data by increasing the counter blower speed

Typical System Resistance CurveReference: www.fantester.com

Theoretical Operating PointSuperimpose Performance curve on Impedance Curve.Intersection of the two curves represents theoretical operating point of the fan.

Theoretical operating pointReference: www.fantester.com

Thermal ResistanceWith the evolution of the personal computer, the cooling of high power components has moved to the forefront of system designOver the years the power dissipation in the PC s microprocessor has been increasing steadilyFor this reason, the use of heat sinks in computers has become more commonBy measuring thermal resistance as a function of free stream velocity, thermal designers can predict the performance of heat sinks in their system and predict the temperature of components

Calculation of thermal resistanceThe airflow chamber is used as the air source for the systemFor a given volume of air drawn through the system temperatures are measuredThermal resistance is calculated by:

where, Tcomponent = Case temperature of component Tambient = Ambient temperature upstream of the heat sink Pcomponent = Power dissipation of component Rthermal = Thermal Resistance*Tcomponent = Tambient + Pcomponent x Rthermal

Calculation of thermal resistance (Contd..)*Graph of Thermal Resistance Vs. Approach velocity is plotted

* Reference: Standardizing heat sink characterization for forced convection by Christian Belady

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