11062015, 09:30 PM  #37  
Join Date: Nov 2004
Location: The plutonium rich regions of Washington State

Re: Questions about UT smoke, lasers, and plasma.
Quote:
If we have a laser beam with intensity I, a smoke particle of radius R will intercept a power P = 2 pi I R^2. At steady state, the solution to the heat conduction equation for a spherically symmetric source in a uniform bath with asymptotic temperature T0 is T = (P/(4 pi K r)) + T0, where T is the temperature at distance r, P is the power being conducted away through the shell of radius r, and K is the thermal conductivity. At r = R, the temperature T will equal the temperature of the smoke particle, and at steady state the conducted power equals the intercepted radiant power T = T0 + (2 pi I R^2)/(4 pi K R) = T0 + (I R)/(2 K). The higher power modern laser weapons put out about 150 kW. From video footage, they appear to be focused into a spot about 3 cm wide  call it 10 cm^2 in area. This gives I = 1.5E8 W/m^2. The thermal conductivity for room temperature (300 K) air is 0.024 W/(K m). Smoke particles typically range in size from 1E8 m to 1E6 m  lets take the geometric average and assume 1E7 m for our typical smoke particle. T0 is room temperature, or 300 K. Plugging all this in, we find that the smoke particle is heated to a temperature of less than 612 K. This is well below the autoignition temperature for reasonable mostlyalreadyburned stuff and way below the temperature where there is any significant evaporation, so the 0.1 micron wide particles will not be burnt up or evaporated by the beam. Any smaller particles will be even cooler, and also will be unaffected. Larger particles might end up igniting or even evaporating  plugging in the numbers for the maximum size (1 micron) of smoke particle we get an upper limit temperature of 3400 K. If the smoke particle actually reached this temperature, it would quickly evaporate if it hadn't burnt up. However, as mentioned, it will not get this hot. A lower limit for the temperature can be found by using the thermal conductivity for air at the upper temperature limit (and noting that the thermal conductivity varies as the square root of the temperature), which gives us a final temperature between about 1000 K and 3400 K. So it looks possible that the largest smoke particles could ignite or evaporate  at the very least they should sparkle red, orange, or yellow from their blackbody radiation. So, final result  most of the smoke will not be burnt through by a modern laser weapon at close focus, but a few of the very largest of the smoke particles might get burnt up. Luke 

Tags 
laser, plasma weapons, ultratech 
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