# Tag Info

11

Beryllium oxide is a very good electrical insulator but at the same time the best non-metal (except diamond) thermal conductor. So to summarize. In general, good thermal conductivity is correlated with good electrical conductivity, but it is not a strict relationship. For example, there is the empirical Wiedemann-Franz law for metals which states that the ...

9

I can think of a few possible ways to approach this. Probably the first thing to look at is the design of the case itself. They generally tend to be made up of thin, flat panels riveted to a frame. In this sort of situation you can get a lot of benefit from the use of a relatively small amount of sound deadening material. This approach is widely used on ...

7

For metals, good electrical conductivity does indeed imply good thermal conductivity. This is known from the Wiedemann–Franz law, which gives the ratio between electronic contribution of thermal conductivity ($\lambda$) and electrical conductivity ($\sigma$) and is proportional to the temperature ($T$). $$\frac{\lambda}{\sigma} = LT$$ This gives the ...

5

If you are interested in knowing why your tea is cooling in different times, you already answered the question yourself. It is due to the different thermal conductivities of the different items you use it to store. $\dot{Q} = A U \Delta T \tag{1}$ is the heat flux you want to know. This gives you a relationship between the heat that is transfered to the ...

5

In the molecular flow region of pressure, the thermal conductivity of an ideal, monatomic gas is obtained by this equation. $$k = \frac{1}{\pi^{3/2} d^2}\sqrt{k_B^3T/m}$$ where $d$ is the collision or molecular diameter and m is the molar mass divided by Avogadro's number. It is independent of pressure. Approximations have been derived for non-monatomic ...

3

Your approach is not an unusual one, engine brakes to measure horsepower usually make use of such a method, but they use much bigger water barrels. To answer your questions: "Can I just fill a tub of water, and divide the output exhaust into 4/5 tubes, take them through a tank of water?" A: Yes you can, but the heat exchange between the water and ...

3

No, it's not meaningful to express it as a ratio. The best insulation is deep insulation everywhere. You also need to consider convection losses, and your ventilation strategy. The attic is usually targetted first, for a combination of several reasons. It's cheap, it's easy, and heat rises, so thoroughly insulating the attic cuts down a lot of conduction ...

3

Well, I would think that two smaller ones rigged up in an "amateurish" fashion (apologies, but that's we both are, right) are going to be worse than just one (sufficiently large) properly mounted heatsink. That giant triangular piece closest to the CPU would probably delay heat transfer -- even if you have excellent cooling on the other side of it. What I ...

3

There can be differences between ice packs, but in practice the differences seem pretty small, and you'll have a hard time finding hard data on different ice packs' cooling abilities. How much ice/ice packs cool is determined by three things: First, the specific heat capacity of a material tells you how much energy it takes to raise 1g of the material by 1 ...

3

Heat is the transfer of energy from a hotter region to a cooler region. The surface in contact with the earth enjoys the benefit of conductive heating. Being cooler, the energy of the warmer earth will be transferred to the earth-bridge, ostensibly until the energy imbalance is removed. The air-bridge is going to become colder due to the colder air and will ...

3

I expect the same material as the coils would be the easiest answer ; nichrome ,Inconel ( 600, 601 or other number) , chromel. There are small differences but any one you can find would be good. That would also be practical as it would be readily available to the manufacturer. Stainless like 304 will probably work but I am uncertain of the many cycles into ...

3

Go to Omega.com to learn all you want to know about thermocouples (TC). I'll give you here a few basics, directed at your questions. TC science is well understood and standardardized. Every type TC is made with it's own precise metal alloy and all TC of the same type will output a voltage uniquely related to the temperature of the TC junction, within ...

3

Your are correct that the smallest surface area will be a limit on the thermal conductivity. Before we dive into that lets look at the over all approach and some other limits. Are your temperatures typical for your laptop on forums or per manufacture? Heatsinks are just lumps of metal so you can easily inspect the quality, but heat pipes are full of a ...

2

The first couple of things that come to mind are metallic 'wools' (ex. steel wool/stainless steel 'scrub pads'), or carbon foams. Metal wools will provide minimal thermal insulation (metal strands conduct, air gaps insulate, so it balances out mostly), but a large amount of vibration dampening 'limp mass.' Carbon foams/aerogels/what not: aerogels are very ...

2

$\dot{Q} = k A \frac{\Delta T}{l}$ $l$ thickness $A$ surface area From this follows that besides other factors the rate of heatflow is proportional to the thermal conductivity $k$. $\dot{Q} \propto k$ Therefore a high thermal conductivity allows for a fast heat transfer, hence your interpretation that a fast change in temperature correlates with a ...

2

Attempting this as a home experiment is unlikely to be feasible. https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator#241Am gives the efficiency of professional prototype designs using Americum as about 2 watts / kilogram of Am. The electrical power you will get from a few micrograms of material will be undetectable (of the order of picowatts)...

2

I have not used Kanthal, but I imagine it has similar properties to nicrome. Basically when you heat the wire up and cool it quickly, it quenches the wire. To reverse this process (or avoid it all together) you need to bring the metal to a specific temperature (without looking it up, red hot is plenty), hold it there for some amount of time, then slowly cool ...

2

First off, the design problem is not trivial and it looks like you have done your homework. On high temperature, small vessels like this you need to consider the 3 dimensional transfer of heat (the wall area is increasing as you move out). Or you can be conservative and much more easily calculate it with all the insulation acting at the outside dimensions. ...

2

As alluded to in the other answers, a most notable exception is diamond. Diamond is an excellent thermal conductor. The thermal conductivity of natural diamond is around 22 W/(cm·K), which makes diamond five times better than copper at conducting heat. At the same time, the electrical resistivity of most diamonds is on the order of 10E11 to 10E18 Ω·m.

2

Stagnate Gas When the gas is stagnate, view the system as though it is an extended surface (from compressor to outlet). Take the heat transfer inside the tube as having no radial component. Develop the differential equation along the tube $z$ as $$\frac{d}{dz} k_G A \frac{dT}{dz} - \frac{P}{R'}\left(T - T_\infty\right) = 0 \\ \frac{1}{R'} = \left(\frac{1}{... 2 There are a lot of variables to consider for a thermal analysis, but none of them require pressure as a boundary condition. As alephzero indicated in their comment, this is function of heat source and sink. Assuming the most basic situation, steady-state conduction, you could use the equivalent resistance equation:$$Q = (T_1-T_2)/R$$Where Q is heat ... 2 This question can only be answered after many tests as to the convective properties of the box, it's geometry with respect to the geometry of the void, ignoring radiation. Because convection which is going to be the major heat transfer mechanism is basically a complex movement of air flowing up in a widening plume, and losing its temperature. Here is a link ... 2 If I get you right, there is a box having a small device that dissipates 30 Watts of heat continuously into the air. The air is blocked into a box, no way in or out. There are no fans/blowers trying to cool the device itself nor circulating the air inside the box. The size of the box is to be defined and the air temperature inside the box has to be figured ... 2 If your temperature sensor is far away from the heat source, the detected temperature may increase even after the heat gun is turned off due to a preexisting temperature gradient evening itself out. 2 The results are telling you that—according to this model—there is no outer temperature that you can apply to obtain an inner temperature as low as 1300 K at steady state for this power output, geometry, and material thermal conductivity (1 W/m-K). If you were to apply ~0 K, this model would predict an inner temperature of 9328+1300=10628 K (at which point ... 2 Consider that ohmic heaters are usually made of thin wire, and that the majority of the current flow going through any wire is close to the surface of the wire. Both these things mean that even if the wire itself might be a poor conductor of heat, that heat has only a very short distance to travel before it reaches the wire surface and gets carried away by ... 2 For maximum power efficiency, you want to match the load impedance (i.e., the heating element resistance) to the source impedance (i.e., the resistance of everything else in the circuit, including the power supply). This is the maximum power transfer theorem. (Volumetric heat generation is J^2\rho, where J is the current density and \rho is the ... 2 The definition of the thermal boundary layer is that of the distance across a boundary layer from the wall to a point where the flow temperature has essentially reached the 'free stream' temperature, T_{0}. This distance is defined normal to the wall in the y-direction The Prandtl number is defined as the ratio of momentum diffusivity to thermal ... 1 Imagine the system below at equilibrium. The energy balances are as follows: At the ball ...$$\epsilon_b T_b^4 =\epsilon_w T_w^4$$At the wall ...$$\epsilon_w A_w \sigma T_w^4 = h_a A_w (T_a - T_w) + \epsilon_a A_w \sigma T_a^4 In these balances, $\epsilon_j$ are emissivities, $A_j$ are areas, $\sigma$ is the Stefan-Boltzmann constant, $T_j$ are ...

1

alright! After all the comments and what was available this is what we ended up designing The inlet flue gas at 140 degrees was 65-75 degrees at outlet after an hour of continuous usage, with outdoor temperature as 25 degrees.

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