My uncle recently told me that a campfire blowpipe (It is used to blow air into the fire so that it continues to burn) works on the Bernoulli's Principle. How is it so?
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asked Apr 18, 2016 at 14:58
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$\begingroup$ do you have a link where it is explained how such a thing works...? If it's so simple as I would imagine it, then I'd suggest that your uncle is wrong... $\endgroup$– IljaCommented Apr 18, 2016 at 15:01
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$\begingroup$ @Ilja: I think the OP means something like this: beaverbushcraft.co.uk/ourshop/…. Bernoulli can approx. be applied there. $\endgroup$– GertCommented Apr 18, 2016 at 15:43
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1 Answer
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If by campfire blowpipe is intended something like the schematic, where the user blows into the left side and the right side is pointed at the fire, then your uncle is loosely correct.
Because of the continuity requirement and assuming air is not very compressible at low pressures and speeds, then the outlet and inlet airspeeds, resp. $v_2$ and $v_1$ are related by:
$$A_1v_1\approx A_2v_2$$
So:
$$v_2\approx \frac{A_1}{A_2}v_1$$
Thus $v_2 > v_1$.
So the campfire blowpipe allows to, from a safe distance, direct a high speed jet of air onto the fire, acc. Bernoulli's Principle, applied here:
$$\frac12 v_1^2+\frac{p_1}{\rho}+gz=\frac12 v_2^2+\frac{p_0}{\rho}+gz$$ Or: $$\frac12(v_2^2-v_1^2)=\frac{p_1-p_0}{\rho}$$ Where $p_1$ is provided by the user and $p_0$ is the atmospheric pressure.
Combined with the equation higher up, $v_2$ could then be estimated.
