Questions tagged [convection]
transport mechanism involving bulk motion of a medium
194
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Rayleigh number for long slit heated at the end
The Rayleigh number is generally described as distinguishing between the convective and coductive regime of heat flow in a fluid. I'm looking at a specific geometry. and it's not clear to me precisely ...
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14 views
Convection current vs current created by an electromotive force acting on a charge
I am wondering if I can use the term 'convection current' for the current created by an electromotive force acting on a charge that sits on some rod that moves through a magnetic field.
The convection ...
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1answer
62 views
Heat Transfer on boiling water
Let's say I'll boil water on the stove from 25°C to 100°C, what heat transfer happens? Is it conduction (heat warms up the pot), or is it convection (causes water molecules to move away from the ...
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1answer
39 views
Is there convection between two volumes of air at different temperatures?
As I have learned in my engineering classes convection takes place when a fluid meets a surface of both different temperatures. My question: can this also be evaluated for two volumes of air at ...
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1answer
42 views
Why do we consider the normal component of velocity in a flux and not the original direction of velocity?
Below is the equation of convective flux of an arbitrary property that is carried by particles. When particles move they carry their property along with them. Given a fixed surface in space, in order ...
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22 views
1D Convective Heat Transfer
I am considering 1D convective heat transfer in a vertical heat pipe with a constant wall temperature Tw, with z varying from 0 to L. Applying the Boussinesq assumption, we arrive at the following ...
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1answer
29 views
What is the temperature profile of hot and cold fluids for a double pipe parallel flow heat exchanger?
In books, they just mention the temperature profile is exponential. Can someone help me come up with the exact temperature profile (the relation) for the hot and cold fluids in a double pipe parallel ...
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1answer
58 views
Is it wrong to derive Log mean temperature difference (LMTD) of a Heat Exchanger, this way?
I tried deriving the LMTD of a parallel flow double pipe heat exchanger by first finding the temperature profiles of the hot and cold fluid and then averaging them over the entire length of the Heat ...
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1answer
34 views
While determining LMTD for a heat exchanger do we assume, the tube wall to be at a constant temperature (no variation along radial direction)?
The derivation of Log Mean Temperature Difference (LMTD) for a Heat exchanger at preliminary level makes some assumptions like, constant specific heats, steady state operation, no changes in kinetic ...
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1answer
18 views
Intuitive explanation of convection in a closed volume
I am trying to wrap my head around what happens when say a hollow cylinder containing a gas is heated from the bottom.
Intuitively I know that the gas in the cylinder will heat up, expand and pressure ...
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18 views
Thermal Boundary Layer
When we study convection in Heat Transfer then there a part where a fluid flows over a plate maintained at a temperature T. In this case heat transfer ocurrs both by conduction as well as convection ...
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67 views
What is the ideal gap between two panes of glass for insulation? [closed]
I have two panes of glass, about a 3/8" thick (.95 cm). Space efficiency and aesthetics ignored, how far apart should they be spaced to maximize heat insulation, assuming the gap contains regular ...
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27 views
Why is the rate of energy conducted in a rod equals the rate of energy released from the surface at the other end?
Assuming energy is released as convection only,we always do questions by equalizing,
$$P_{\text{conducting}} = Q/t = KA (T1 - T2)/L = eA(dT).$$
Assume the system is at Steady State. Why does this ...
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27 views
Why does bologna inflate radially when on stove?
I noticed that when heating up bologna in any way it inflates at the center, while other stuff tends to let the air escape sideways, or inflates in different places, like an egg or sausage. Nothing so ...
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1answer
53 views
Navier-Stokes Convective term divergence of tensor is a row vector
Consider the convective term from the Navier-Sokes equation (e.g. Navier–Stokes momentum equation (conservation form) from https://en.wikipedia.org/wiki/Navier%E2%80%93Stokes_equations):
$$
\nabla \...
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1answer
28 views
Drift velocity in advection-diffusion equation
The advection-diffusion equation is given by
$$\partial_{t}\rho=-\nabla\cdot\left(\rho\mathbf{v}_{drift}\right)+\nabla\cdot\left(D\nabla\rho\right)\equiv-\nabla\cdot\left(\rho\mathbf{v}_{current}\...
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135 views
Heat Generation in Watts per cubic meter - how to apply this to a volume?
Water is running through a pipe. The pipe is heated uniformly in order to heat the water running through the pipe. I want to calculate how much heat needs to be generated by the pipe in order to heat ...
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27 views
How does the speed/rate of convection currents in water in a tube affect the change to temperature of water at the upper end of the tube?
I have done some experiments in which I heated a burette filled with water by applying a preheated soldering iron to approximately the middle of the burette and measured how the temperature of water ...
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9 views
Convection/conduction simulation run from equal temp, but losing heat
I was investigating the effects of heated wall materials on outdoor air temperature and indoor air temperature with a certain software. The system is as the following:
1x1x1 meter cube of outdoor air
...
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32 views
Hypothetical: generating an inadvertent and transient magnetic field in a massive biodome
I am doing a thought experiment for a sci-fi novel and have a physics related question. I do have some elementary knowledge of physics from college. Before I ask my question, I need to establish a ...
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34 views
Can one model fluid flow only by using the convection–diffusion equation?
Modeling fluid flow in a coil seems very complicated, especially for turbulent flow. Having Navier-Stokes equations seems to be the right way to go, but I was wondering... can one use the convection-...
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71 views
How to obtain a solution for the following IBVP?
I am trying to solve the following advection-diffusion equation for transient flow conditions for radial flow.
The governing equation is as follows.
$$\frac{\partial T}{\partial t} = \frac{\partial^2 ...
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1answer
68 views
Materials to Insulate Against Convection, But Not Radiation
Are there any materials which could effectively insulate against convection (and/or conduction), that don't insulate against radiation?
Specifically, I want to make a water-heater that uses solar ...
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35 views
Quickest way to dry wet floor
I have spilled around one liter of water to wooden floor. I'm afraid wood will deform and pop out.
What would be quickest way to dry it? I have wiped out what I could, but some water passed between ...
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19 views
Convective Air Velocity
Consider there is a hot plate which causes natural convection in a closed environment of air over the hot plate. Is there any exact formula to find velocity of air flow? If there is please cite the ...
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Convection powered bladeless fan style ventilation?
So as I understand it Bladeless fans use the Coandă effect of air hugging the walls of an expanding cylinder to creat a lower pressure zone that pulls additional air in through the center.
My question ...
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3answers
159 views
Why doesn't convection break conservation of energy?
assumption: Warmer gas rises. (under gravity)
assumption: A volume of gas cannot have uniform temperature, because gas with fully uniform temperature (all molecules having the same speed) has lower ...
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1answer
405 views
Why does a paper spiral spin when convection happens?
It is a classic experiment demonstrating convection. I understand that convection takes place and heats up the air. The heated air rises and pushes the spiral from the bottom. However, how does the ...
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1answer
209 views
Calculating heat transfer coefficient from cooling rate of a block?
If I have a cube of metal with known surface area and time to cool from $T_2$ to $T_1$, determined experimentally. Can I calculate the convective heat transfer coefficient, h of this situation from ...
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1answer
75 views
Cooling something by applying a decreasing temperature
I've recently come across Fourier's Heat Equation: $k \cdot \nabla^2 \; T = \rho c \cdot \dot{T}$ and the solution to Newton's Law of Cooling: $T(t)=\left(T_\text{initial} - T_\text{cool}\right) \cdot ...
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2answers
48 views
Heat transfer coefficient for lumped-capacity convection
I am using Lienhard & Lienhard's A Heat Transfer Textbook (freely available here) to study Heat Transfer.
On page 19, the heat flux is written as $q=\bar{h}\cdot{(T_{body}(t)-T_\infty)}$, where $\...
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3answers
63 views
Relation between volume of a room and temperature inside
I don't know whether it is based on some people's assumptions or it is a fact that the smaller the room, the hotter it is (say, during summer), considering the heat is only supplied from outside the ...
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1answer
74 views
Determination of the thermal resistance of a molding tool with cooling channel
This is a section of cooling channels in injection moulding tools.
I have tried to calculate the thermal resistance in this section with 2 methods, one analytical and one numerical.
analytical ...
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2answers
2k views
Calculating heat loss to surroundings from a cube suspended in air?
If you have a cube that is suspended in the middle of the air, how would you calculate the heat loss to its surroundings?
For this example, I am assuming the following:
Dimensions: 1m×1m×1m
...
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3answers
34 views
Temperature drop in the interface between air and plane surface
I’ve searched a lot but I couldn’t find any answer. What is the reason behind the temperature drop between the air-surface interface?
Why isn’t it just like:
And what even causes this drop to be non-...
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0answers
25 views
Exponent of Lewis number in coupled heat-mass transfer problem
I am writing a small model of the coupling of heat and mass transfers in the nose and mouth. By solving the heat and mass balance equations, Lewis number expression appears that allows for coupling ...
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3answers
157 views
What will happen if we boil water in the absence of gravity?
We know that, Convection is the mode of heat transfer due to the motion of individual molecules of a fluid (liquid or gas). The motion of the constituent particles arises due to difference in ...
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2answers
69 views
How is fin efficiency a useful quantity for a fin's heat dissipation performance? [closed]
I mean, it's the ratio of actual heat transfer to heat transfer if the material had infinite conductivity.
How is it of any practical, or even theoretical use? How does it help, in saying one fin ...
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1answer
410 views
Is a Microwave oven without a faraday cage a health hazard?
Microwave ovens have something of a Faraday-cage/shield to keep from leaking (emitting) EM radiation.
What would happen if the shield or the door was removed? No doubt it would be less efficient as ...
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41 views
Limits to natural convection in a non-infinite medium
I'm trying to understand the temperature profile of a fluid in an underground vertical pipe without flow. The fluid is subject to a boundary condition of the form:
$$Q = h[T_e(z) - T],$$
where $Q$ ...
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1answer
181 views
Effect of volumetric flow rate on convection and radiation heat fluxes
Say we have two hot objects of the same material but of different width and same thickness.
The objects are moving horizontally, have initially the same temperature and are cooled by a gas blown from ...
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1answer
80 views
The convective analog to Fourier's equation of conduction?
The differential form of the thermal conduction law is given by
$J=-\kappa\nabla T$
where $\kappa$ is the thermal conduction coefficient, or in the one-dimensional case,
$J=-\kappa \frac{\partial ...
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3answers
2k views
Conduction, convection, radiation: Does evaporation count as one of those?
The forms of heat transfer are traditionally described as conduction, convection, and radiation. Is evaporation (or any other change of state) counted as one of those forms? Or is it considered its ...
2
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1answer
98 views
The solution to the non-linear convection equation
The non-linear convection equation
$$u_{t} +uu_{x}=0$$ admits implicit solutions of the form $$u=f(x-ut).$$
How does one interpret this solution intuitively? Is there an example of a solution of this ...
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1answer
65 views
Are there no convection currents in ice-boxes and aerogel?
Technologies dealt with
Ice-boxes have been used for a long time to prevent ice from melting for a longer time than keeping it in open. Aerogel is still an emerging technology (I came across it on ...
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3answers
156 views
From kinetic theory - why does heat rise?
I know the explanation from fluid elements and densities however I don't find it satisfying. Is there an explanation from kinetic theory?
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132 views
Density of ice < water?
In the situation of an ice cube placed in water and eventually melting, it is supposed that natural convection is the process in which heat transfer from water to ice occurs.
However, natural ...
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1answer
117 views
What is oscillatory convection?
I come across this term in a lot of papers which talk about convection and instabilities but could not find a proper explanation of what oscillatory convection is. Is it the fluid which is oscillating ...
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2answers
284 views
Does the Boussinesq approximation imply 'incompressible' flow?
Does the Boussinesq approximation imply that compressible inertial effects in the flow can be neglected (i.e. density can otherwise be treated as constant)?
It seems that it must be the case and ...
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101 views
How much conduction, radiation and convection influence in a thermos flask?
It has been thoroughly repeated that thermos flasks prevent heat loss by minimizing heat conduction, radiation and convection from the bottle to its ambient. This has been answered in this SE. However,...
