标签: heating

We tend to picture lasers, regardless of power rating, as highly focused, coherent light sources. After all, one of the virtues of the laser is that its beam doesn't spread, so it can be used for targeted illumination (such as scanner or rangefinder), or high-intensity localized heating (cutting or welding metals), to cite just a few of their thousands of applications. (Historical side note: when the laser was first demonstrated, one pundit wagged "it was a solution looking for a problem to solve," and we know how that quip turned out!)   But using lasers for area heating seems to be a contrary to their virtues. After all you can heat with IR lamps, microwaves, heated air, electric coils, induction coils, or gas-fired burners, to cite just a few possibilities. Why would you go to the complications of using lasers unless you had no alternative?   That's why I was surprised when I saw the story on the benefits of "photonic" heating in Laser Focus World, "High-power VCSEL arrays make ideal industrial heating systems." By setting up an array of hundreds of vertical-cavity surface-emitting lasers (shown below), you not only obviously get a different source of heat, but you attain some other unique operating advantages that are non-obvious and beneficial. Yes, the author's company (Photonics Aachen, part of Philips Photonics ) makes this system and so he is somewhat biased, but nonetheless, it's worth seeing what he has to say.   A one or two-dimensional array of VCSELs can be used as tightly spaced, easily modulated high-intensity heat source. (Source: Philips Photonics)   In contrast to conventional edge-emitting laser diodes, the collection of vertical-emitting laser diodes (each with a diameter of 30-40 μmeter) can be fabricated in one pass of wafer processing, including test, with about 500 VCSELs per mm 2 of a die. Since each laser emits 1 to 10 mW, a 2 × 2 mm chip array holding 2,000 VCSELs can emit over 20 W of infrared power — that's impressive power density for this technology. These arrays can be connected in series, so arrays of hundreds of watts and even kW have been built. While heat sinking of the die is an issue, it’s a manageable one, the author claims.   All this is impressive, but why bother when you can use standard halogen lamps, for example, to get the IR heating? First, the VCSEL IR brightness is 100 to 1,000 greater than halogens, with a lifetime of over than 10,000 hours, the author says (and I'll have to accept those numbers for now).   But the advantages of VCSEL-based heating go beyond just density and lifetime. The VCSEL array can be switched on and off in milliseconds for precise dosing control, since it does not have the thermal lag of a halogen emitter or similar sources. Also, the VCSEL array is well suited to highly targeted, localized zones, where the material to be heated may not be homogenous, with some areas needing more or less heat or specialized heat-treating patterns are preferred (think of PC boards to be wave soldered and loaded with different-size/mass components). Even if this level of control is not a requirement, the output power and thus heat is tightly focused, so that the entire oven does not have to be heated; only the part of the material that needs treatment. Further, unlike bulbs, the VCSEL's output wavelength does not change as it is dimmed, which means the target material's thermal absorption characteristics are unchanged, a factor in precision situation. Dimming control is relatively easy, as the VCSELs are driven by a controllable DC current.   I don’t have a need to heat anything with VCSELs, of course, nor do I fully understand what downsides of this heating approach. Still, reading about this application reminded me that what we often consider a key attribute of a technology can sometimes be less relevant, while its downside can become a virtue. While we prize lasers for their ability to deliver highly focused beams of photonic power, and use ever-bigger single-source lasers to deliver increasingly powerful punches, some out-of-the-box thinking shows that an aggregation of many small lasers as heat sources can be used to advantage for some applications.   Have you seen other cases where contrary thinking has solved a power problem or turned a thermal weakness into an advantage? Have you ever done this?

We tend to picture lasers, regardless of power rating, as highly focused, coherent light sources. After all, one of the virtues of the laser is that its beam doesn't spread, so it can be used for targeted illumination (such as scanner or rangefinder), or high-intensity localized heating (cutting or welding metals), to cite just a few of their thousands of applications. (Historical side note: when the laser was first demonstrated, one pundit wagged "it was a solution looking for a problem to solve," and we know how that quip turned out!)   But using lasers for area heating seems to be a contrary to their virtues. After all you can heat with IR lamps, microwaves, heated air, electric coils, induction coils, or gas-fired burners, to cite just a few possibilities. Why would you go to the complications of using lasers unless you had no alternative?   That's why I was surprised when I saw the story on the benefits of "photonic" heating in Laser Focus World, "High-power VCSEL arrays make ideal industrial heating systems." By setting up an array of hundreds of vertical-cavity surface-emitting lasers (shown below), you not only obviously get a different source of heat, but you attain some other unique operating advantages that are non-obvious and beneficial. Yes, the author's company (Photonics Aachen, part of Philips Photonics ) makes this system and so he is somewhat biased, but nonetheless, it's worth seeing what he has to say.   A one or two-dimensional array of VCSELs can be used as tightly spaced, easily modulated high-intensity heat source. (Source: Philips Photonics)   In contrast to conventional edge-emitting laser diodes, the collection of vertical-emitting laser diodes (each with a diameter of 30-40 μmeter) can be fabricated in one pass of wafer processing, including test, with about 500 VCSELs per mm 2 of a die. Since each laser emits 1 to 10 mW, a 2 × 2 mm chip array holding 2,000 VCSELs can emit over 20 W of infrared power — that's impressive power density for this technology. These arrays can be connected in series, so arrays of hundreds of watts and even kW have been built. While heat sinking of the die is an issue, it’s a manageable one, the author claims.   All this is impressive, but why bother when you can use standard halogen lamps, for example, to get the IR heating? First, the VCSEL IR brightness is 100 to 1,000 greater than halogens, with a lifetime of over than 10,000 hours, the author says (and I'll have to accept those numbers for now).   But the advantages of VCSEL-based heating go beyond just density and lifetime. The VCSEL array can be switched on and off in milliseconds for precise dosing control, since it does not have the thermal lag of a halogen emitter or similar sources. Also, the VCSEL array is well suited to highly targeted, localized zones, where the material to be heated may not be homogenous, with some areas needing more or less heat or specialized heat-treating patterns are preferred (think of PC boards to be wave soldered and loaded with different-size/mass components). Even if this level of control is not a requirement, the output power and thus heat is tightly focused, so that the entire oven does not have to be heated; only the part of the material that needs treatment. Further, unlike bulbs, the VCSEL's output wavelength does not change as it is dimmed, which means the target material's thermal absorption characteristics are unchanged, a factor in precision situation. Dimming control is relatively easy, as the VCSELs are driven by a controllable DC current.   I don’t have a need to heat anything with VCSELs, of course, nor do I fully understand what downsides of this heating approach. Still, reading about this application reminded me that what we often consider a key attribute of a technology can sometimes be less relevant, while its downside can become a virtue. While we prize lasers for their ability to deliver highly focused beams of photonic power, and use ever-bigger single-source lasers to deliver increasingly powerful punches, some out-of-the-box thinking shows that an aggregation of many small lasers as heat sources can be used to advantage for some applications.   Have you seen other cases where contrary thinking has solved a power problem or turned a thermal weakness into an advantage? Have you ever done this?

About once a month, I check my car tyres, since correct pressure is necessary for good car handling, a smooth ride, and good gas mileage. When I checked my front tyres recently with my Topeak digital gauge (image below), one was at the correct pressure (30 psi), but the other was higher, at about 33 psi. What puzzled me was that they had both been at 30 psi the previous time. I know tyres can lose pressure, but I had never heard of a case where the pressure increased on its own.     This inexpensive digital-pressure gauge is a pleasure to use: It reads up to 160 psi/11 bar (useful for bike tyres and suspensions) to three significant figures; can be switched among psi, Bar, or kg/cm² readings; and handles Presta and Schrader valves -- a big improvement over the old "pencil-type" mechanical tyre-pressure gauges.   I gave this some thought and saw only two possibilities at first: Someone was playing a practical joke on me (very unlikely); or my previous reading for just that one tyre alone was in error (also unlikely, as all tyres were measured twice, and at the same time).   Then I looked at the car and saw that the tyre that read high was in full sunlight, while the other was in the shadow, and a black tyre certainly does heat up from solar radiation. Mystery solved -- or maybe not. I pulled out my custom-made "back of the envelope" pad and did a quick calculation using the ideal gas law :   P × V /T = K or P = K × T / V   ...where P is pressure, V is volume, T is the absolute temperature, and K is a constant, which depends on the amount and type of gas (here, the value is irrelevant).     My hand-made "back of the envelope" pad reminds me that doing quick, rough estimates is often a good first step to understanding the parameters of a problem.   Thus if the pressure I measured was about 10% higher than the original, and the volume was constant, then the temperature of the air in the tyre also should have gone up about 10%. The "cold" ambient temperature was about 77°F (25°C) or about 300K (remember, this is a rough estimate we're doing), so the delta rise would be 30K (30°C), or about 55°F.   Then I worried that perhaps a change in the tyre's volume would affect my estimate, but I realized it was a non-issue for two reasons. First, a car tyre is not an easily expandable balloon; it is a rubber enclosure restrained by steel-wire belts. Therefore, its volume stays fairly constant, especially for modest variations around a nominal value (this is a type of assumption we often use in many simplified models).   Second, even if the tyre did expand slightly due to the increase in internal pressure, that would actually cause a decrease in the resultant pressure -- again, the gas law. (I recall seeing a complex differential equation embodying the relationship among a tyre's construction, pressure, and volume, for more advanced modeling.)   Was solar heating the answer to my mystery? I don’t know, as I have no way of measuring the internal air temperature. I suppose I could do some thermal modeling, or even use an application such as COMSOL Multiphysics for a thermal/mechanical simulation, but it's not worth the effort.   So the question of sunlight heating the tyre and raising the pressure remains a slightly open mystery. My "gut" tells me that a 30°C/55°F rise for a black-rubber tyre in full sunlight is possible, but that's where I have to stop.   Do you think it was solar-heating effect? Can you think of any other causes? Have you ever had a similar "simple" measurement mystery, where you are not sure of the actual cause of the observed effect?

I check my car tyres about once a month just like many people, since correct pressure is necessary for good car handling, a smooth ride, and good gas mileage. When I checked my front tyres recently with my Topeak digital gauge (image below), one was at the correct pressure (30 psi), but the other was higher, at about 33 psi. What puzzled me was that they had both been at 30 psi the previous time. I know tyres can lose pressure, but I had never heard of a case where the pressure increased on its own.     This inexpensive digital-pressure gauge is a pleasure to use: It reads up to 160 psi/11 bar (useful for bike tyres and suspensions) to three significant figures; can be switched among psi, Bar, or kg/cm² readings; and handles Presta and Schrader valves -- a big improvement over the old "pencil-type" mechanical tyre-pressure gauges.   I gave this some thought and saw only two possibilities at first: Someone was playing a practical joke on me (very unlikely); or my previous reading for just that one tyre alone was in error (also unlikely, as all tyres were measured twice, and at the same time).   Then I looked at the car and saw that the tyre that read high was in full sunlight, while the other was in the shadow, and a black tyre certainly does heat up from solar radiation. Mystery solved -- or maybe not. I pulled out my custom-made "back of the envelope" pad and did a quick calculation using the ideal gas law :   P × V /T = K or P = K × T / V   ...where P is pressure, V is volume, T is the absolute temperature, and K is a constant, which depends on the amount and type of gas (here, the value is irrelevant).     My hand-made "back of the envelope" pad reminds me that doing quick, rough estimates is often a good first step to understanding the parameters of a problem.   Thus if the pressure I measured was about 10% higher than the original, and the volume was constant, then the temperature of the air in the tyre also should have gone up about 10%. The "cold" ambient temperature was about 77°F (25°C) or about 300K (remember, this is a rough estimate we're doing), so the delta rise would be 30K (30°C), or about 55°F.   Then I worried that perhaps a change in the tyre's volume would affect my estimate, but I realized it was a non-issue for two reasons. First, a car tyre is not an easily expandable balloon; it is a rubber enclosure restrained by steel-wire belts. Therefore, its volume stays fairly constant, especially for modest variations around a nominal value (this is a type of assumption we often use in many simplified models).   Second, even if the tyre did expand slightly due to the increase in internal pressure, that would actually cause a decrease in the resultant pressure -- again, the gas law. (I recall seeing a complex differential equation embodying the relationship among a tyre's construction, pressure, and volume, for more advanced modeling.)   Was solar heating the answer to my mystery? I don’t know, as I have no way of measuring the internal air temperature. I suppose I could do some thermal modeling, or even use an application such as COMSOL Multiphysics for a thermal/mechanical simulation, but it's not worth the effort.   So the question of sunlight heating the tyre and raising the pressure remains a slightly open mystery. My "gut" tells me that a 30°C/55°F rise for a black-rubber tyre in full sunlight is possible, but that's where I have to stop.   Do you think it was solar-heating effect? Can you think of any other causes? Have you ever had a similar "simple" measurement mystery, where you are not sure of the actual cause of the observed effect?