AirFlow Calculations/ Anemometer anyone?

  • Context: Undergrad 
  • Thread starter Thread starter CstmIndctn
  • Start date Start date
  • Tags Tags
    Airflow Calculations
Click For Summary

Discussion Overview

The discussion revolves around calculating airflow (CFM) using an anemometer in the context of a truck's air intake system. Participants explore the challenges of measuring airflow through various sections of the intake, including the impact of different diameters and the resistance introduced by filters.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant seeks to calculate CFM for an air intake system, noting the complexity due to varying diameters (8" entrance, 4" exit, and 2" hose).
  • Another participant points out the need to convert area measurements to square feet, indicating a potential error in calculations due to using square inches.
  • Some participants discuss the importance of measuring airflow at different sections, suggesting that mass flow remains constant but velocities will change with area adjustments.
  • A participant expresses uncertainty about measuring airflow at the filter housing, considering it may not provide accurate results due to resistance.
  • There is a discussion about comparing the airflow of a new intake system to a stock system, with one participant proposing to simulate airflow conditions to measure performance.
  • Questions arise about the effects of pipe diameter changes (e.g., from 6" to 4") on airflow calculations, with one participant seeking clarification on how to approach this issue.
  • Another participant inquires about the type of anemometer being used, distinguishing between handheld and automotive-installed devices.

Areas of Agreement / Disagreement

Participants express various viewpoints and uncertainties regarding measurement techniques and calculations, with no clear consensus reached on the best approach to determine airflow or the effects of diameter changes.

Contextual Notes

Participants mention specific measurements and conditions (e.g., airspeed at 50 mph, distances for measurement) but do not resolve the implications of these factors on the overall calculations.

Who May Find This Useful

This discussion may be useful for automotive enthusiasts, engineers, or anyone interested in airflow dynamics and measurement techniques in intake systems.

CstmIndctn
Messages
10
Reaction score
0
AirFlow Calculations/ Anemometer anyone??

Hello all,
I am trying to solve a problem with my anemometer. I am not sure if there is anyone here that has used one but here goes. My purpose is to calculate CFM of a particular AIR INTAKE of my truck. The problems lies in that CFM= Air Velocity Ft/Min X Area ft^2...with Area= PIE * (r^2)...however my intake is 8" diameter at the entrance, then the filter, then a 4" exit...The air supply I am using is (shopvac) 2" diameter hose...what do I use as the area?

I have a chart that says @ 4500 feet per minute on a 4" diameter is 390cfm

2nd question-- What am I doing wrong if I do 3.14 (2^2)= 12.56 Area

CFM= 4500 * 12.56= crazy big number which is not 390cfm =0(
 
Physics news on Phys.org
The last problem first...you need to convert your area value to square feet. You can not use square inches. You're a factor of 144 off.

As far as the first problem, you can choose any section. The idea being that the mass flow is going to be the same through all of the parts. That being said, the velocities will change when there is a change in area. So, if you're interested in the volumetric flow rate, select the section of the shop vac. Take your measurements in the shop vac tube. Then you can calculate the volumetic flow rate in that section. From there you know you'll have the same flow rate at other locations. Just adjust the velocity based on the new area. The filter housing will probably not be a great spot for measurements.
 
Yea wow I am retarded...as far as measuring at the filter housing goes. I am tryin to determine the difference in my intake vs the stock intake. I figure the only was is to hold the air supply a certain distance away (approx 9 inches away gives a 50mph wind) and then measure the flow rate threw the air filter and housing, etc to see how much the actual engine is getting. I know the calculations to determine the Volumetric Efficiency and how much CFM the engine needs at a specific RPM (Volumetric FlowRate)...how else would I determine which intake flows better other than this method?
 
Last edited:
Amateur input here. (Sorry, Fred, but you know that I can't help chirping up when something bugs me.)
Cst(etc.), just make sure that you take the flow rate of the shop vac when it's hooked up as opposed to free-flowing. With a filter in that housing, you'll get some resistance that will probably slow it down a bit. (Or not... Fred?)
 
Danger...Yes the air filter gives resistance, but that is the whole point of creating a intake is to make one that supports more air and supports the CFM of a better filter (K&N) therefore, I would have to compare the air flow through the stock housing and filter to the new air filter and housing setup to see how much more CFM it will flow over stock or if it doesn't create one that will, etc...thats why I was asking what to calculate the area of since in essence I am measuring the airflow through the bigger housing and not the air supply source...

I have determined that the air supply (shop vac) has a velocity of approx 50mph @ 9inches from the point of measurement (where I hold the anemometer) therefore, I hold the air supply 9 inches from the entrance of the airfilter/housing (simulating 50mph wind speed) and hold the anemometer on the exit side of the airfilter/housing (after the actual filter) and then I would get the CFM that is capable of flowing threw that setup @ 50mph...correct? (Fred) only problem is now is trying to calculate the actual CFM...anyone ever use a anemometer? Its a lil tricky
 
At this point, I must turn everything over to Fred. The closest that I ever got to using an anemometer was reading the airspeed indicator on a Cessna, and that won't be of any help to you.
 
Well thanks for the input at any rate ;0)...

Also, One more question when a 6" pipe shrinks down to a 4" on a bend...how would I calculate the effect of this? Or would it even matter assuming I started using this device correctly =0)
 
CstmIndctn said:
Yea wow I am retarded...as far as measuring at the filter housing goes. I am tryin to determine the difference in my intake vs the stock intake. I figure the only was is to hold the air supply a certain distance away (approx 9 inches away gives a 50mph wind) and then measure the flow rate threw the air filter and housing, etc to see how much the actual engine is getting. I know the calculations to determine the Volumetric Efficiency and how much CFM the engine needs at a specific RPM (Volumetric FlowRate)...how else would I determine which intake flows better other than this method?
Silly question, but are you using a handheld anemometer or is it an automotive part that is preinstalled in your intake assembly?
 
Handheld
 

Similar threads

Undergrad Equalizing airflow through multi-hole orifices
  • · Replies 3 ·
Replies
3
Views
2K
A few questions about airflow and filters...
  • · Replies 5 ·
Replies
5
Views
3K
Comparetto type Air Cleaner/Filter: What causes its increased airflow?
  • · Replies 5 ·
Replies
5
Views
3K
Undergrad Calculating Force and Efficiency of Fans Using Airflow Measurements
  • · Replies 5 ·
Replies
5
Views
2K
Undergrad Can someone help calculate pressure drop ?
  • · Replies 3 ·
Replies
3
Views
2K
Undergrad How Do You Calculate Air Pressure with Varying Flow Rates and Orifice Sizes?
  • · Replies 6 ·
Replies
6
Views
3K
Do Dual Fans Increase Radiator Efficiency More Than a Single Larger Fan?
  • · Replies 6 ·
Replies
6
Views
7K
Help Me Design a Motorcycle Airbox for Charity Racing Series
  • · Replies 3 ·
Replies
3
Views
3K
Undergrad Issue calculating water pressure due to the constriction of water flow
  • · Replies 9 ·
Replies
9
Views
2K
Underground pump vault airflow considerations
  • · Replies 15 ·
Replies
15
Views
4K