propane flow through aperture

I'd like to figure out how much propane flows through the tip in a burner I built, in order to determine how much heat it delivers. The tip I'm using now is 0.8" long with an inside diameter of .030" and it has propane coming in at 3.5 psi from the regulator. The propane jet shoots out into a tube and pulls in air to mix with it, the way a weedburner or plumber's torch does. Then the hot exhaust goes into a very efficient heat exchanger that transfers the heat to a diesel engine's coolant. I'm trying out different pressures and aperture sizes. I don't know enough about hydrodynamics to go about calculating flow based on pressure and aperture. I don't know if the flow in the tip is laminar or turbulent, or whether it can be modeled as incompressible for purposes of applying Bernoulli equations. Perhaps there's even a table somewhere listing the flow for different pressures and apertures sizes, in which case I wouldn't have to do any calculations at all? Any help much appreciated.

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Here is an good article on using standard test methods to test your prototype using ISO 6358.

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The only good way I know of predicting flow through something is to use ANSYS CFX. The problem is that the hole size you calculate will have a K factor (air sticks to the sides of any hole) which will be difficult to approximate unless it is a round hole which you can look up.

If you know A1 area of the throat and V1 velocity at the throat then you can calculate V2 velocity at the nozzle if you know A2 area of the nozzle.

A1 and A2 should be multiplied by their respective K factors. A1*V1=A2*V2

The flow at the nozzle Q2 will be A2*V2. The problem is that you kind of have to know Q1 to get Q2 so it is kind of a vicious circle without flow bench testing.

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Don't reinvent the wheel. Homebuilders of metal working furnaces use mig screw in nozzles as a cheap available standard nozzle for propane burners.

Google Reil, propane burner blacksmith furnace

They expect to get about 15 kBTU/hr out of a 30 mil MIG nozzle at

3.5PSI 24kBTU/hr for a 35 mil, 35kBTU/hr for a 45 mil, 63kBTU/hr for a 116 mil nozzle all at 3.5 psi which would be low for these. [Take the heating value of propane as 20 kBTU/Lb to get the mass flow rate estimate] Nozzles choke when the local speed goes sonic. That's at pressures about 10X higher.


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Brian Whatcott

I did use a mig tip.

Thanks for pointing me in the right direction. I found graphs at

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I *could* tell you how to calculate the flow, but then you'd never learn anything. Besides, I'm too cranky to dig out my thermo book and see if I'm right. So just MEASURE the flow and be done with it! Run the nozzle under the edge of a jar (or suitable transparant cylinder) filled with water and inverted in a larger pan of water, and allow the propane to flow for a measured number of seconds. Do this ten times and take the average. Use cold water to reduce error created by the vapor pressure of the water. This won't give you a dead-on result, but it will be darned close.

Another way would be to fill a balloon or plastic bag of known volume and measure the number of seconds to fill it. Less accurate to do it this way, but not as much mess.

Sometimes ya just gotta try NOT to think, and just measure your way to a solution. I bet you can come up with other ways to measure the volume/flow rate of gas after these tow small hints.

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'lektric dan

Why on earth would you rely on CFX (or any CFD code) to predict flow through a tube!? CFD is for complex hard problems that have not been tested to death and emperically correlated by hundreds of people in the past.

Regarding the equations you provide are valid for INCOMPRESSIBLE flow. Propane at the pressure ratio mentioned by the poster mentions has a mach number high enough the definately not be incompressible (~0.6).

Furthermore the reason you have a "viscous circle" is that you've used the same equation (incompressible conservation of mass) for two locations. This is fine if you know the flow (or velocity). However, the poster provided the pressure drop so you need an equation that relates the pressure drop to the mass flow. For compressible flow through a tube of the dimensions described the proper analytical model is isentropic compressible flow. This model could be improved by applying a discharge coefficient from previous testing of similar geometry or actual testing of the fabricated nozzle. However for the length/diameter describe the discharge coefficient should be very close to 1.0 meaning isentropic flow equations are valid.

There are many online isentropic compressible flow calculators available. Just be sure they allow you to plug in the properties of propane (use Google to find the gas constant and specific heat ratio for propane). Here's a simple calculator at NASA:

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