Questions tagged [convection]
transport mechanism involving bulk motion of a medium
247
questions
2
votes
0
answers
38
views
Is stoßlüften - quick exchange of air through windows actually more efficient than slow exchange through a wall ventilation fan?
In German areas with near air-tight homes, it is recommended that full house ventilation is performed by stoßlüften - opening windows fully to exchange as much air as possible in shortest amount of ...
11
votes
2
answers
2k
views
Does opening both windows in a room cool the room twice as fast?
It's getting colder where I live (~40 degrees Fahrenheit). The place where I live (college dormitories) pumps the heat up into the 80's, so I keep the windows open most of the time. I noticed that, ...
0
votes
0
answers
73
views
Relation between Sherwood and Peclet numbers
What's the relation between Sherwood number and Peclet number for mass transfer in a rectangular channel with fully developed laminar flow?
0
votes
1
answer
33
views
Two Particles with identical temperatures at different points in the atmosphere
Quick question with probably a simple answer. For context, I am currently in undergraduate classical mechanics studying potential energy. My question is, if a conservative force is one in which is ...
0
votes
0
answers
17
views
Forced convection and active temperature?
I performed anisothermal CFD computations with large eddy simulation.
Is the use of the temperature as an active scalar relevant when performing a forced convection RANS or LES computation (Ri<0....
0
votes
0
answers
25
views
Is Newtons Law of cooling applicable for all forms of heat loss? [duplicate]
Can heat loss through Conduction, Convection and Radiation be explained by Newton's law of cooling?
Or it is just applicable for heat loss through Convection and Radiation?
0
votes
0
answers
19
views
How could we caluculate the amount of heat transferred through convection when a fluid flows over a curved surface?
In the case of convection heat transfer during fluid flow over a flat plate , we deduce the governing equations based on conservation of mass , momentum and energy, using this equations we find the ...
1
vote
1
answer
129
views
What part of my oven should be hottest, top or bottom? [closed]
My oven has the heating element in the bottom. So the bottom shelf is more directly exposed to it, and should be the hottest shelf.
But hot air rises, so the top shelf should have the hottest air, and ...
1
vote
0
answers
19
views
Designing and optimizing a heatsink given tight constraints [closed]
I am trying to design a heatsink as part of a broader system and deturmine two things:
Given the size constraints of the system, commercially available fans, and heat dissipation requirements, is it ...
0
votes
0
answers
36
views
Fluid viscosity, mass diffusion and Navier-Stokes equation
With the increase of fluid viscosity, mass diffusion of a fluid decreases. Then how the diffusion term in Navier-Stokes equation has a dominant effect at high viscosity? Also how the mass convection ...
0
votes
1
answer
91
views
Convective term in Navier-Stokes
In the Navier-Stokes equations, there's a well-known convective term of the form:
\begin{equation}(\mathbf{v}\cdot\nabla)\mathbf{v}\end{equation}
I'm not able to understand it. As far as I know, the ...
9
votes
2
answers
804
views
Is there a difference in the 'quality' of a gas if it's heated by electromagnetic radiation as opposed to conduction/convection?
According to this link, "The wavelength at which the $O_2$ molecule most strongly absorbs light is approximately $145$ nm."
According to this link, that's in the ultraviolet range of the ...
2
votes
1
answer
62
views
Does warmer air rise significantly to the top of a small container?
Suppose you have a small container, say 2 cubic centimeters (1cm x 1cm x 2cm), positioned so the 2cm side is vertical, and there is a tiny light bulb or heating element at the very top.
Is it the case ...
2
votes
0
answers
51
views
How can the Boussinesq Approximation simplify the general form of the temperature equation for fluids? [closed]
In Transport Phenomena by Bird, Stewart, Lightfoot (2nd edition), on page 337 (11.2–5), the equation for the change in temperature is derived:
$$
\rho\hat{C}_{p}\frac{DT}{Dt}=-\left(\nabla\cdot\mathbf{...
1
vote
0
answers
17
views
Convection in stratified atmosphere
In the stability analysis of stratified atmosphere in the presence of gravity, we consider a blob of atmosphere and perturb it (move it up by a small height $\Delta z$). Initially, the blob had the ...
1
vote
1
answer
18
views
Star convection gradient dependencies
In star convection you can work with radiative and adiabatic temperature gradients, that for example the radiative one can be defined as,
$$\nabla_{rad}=\frac{3\kappa L P}{16 \pi a c G m T^4}$$
This ...
0
votes
0
answers
26
views
Velocity field of a fluid in a porous pipe
Consider the situation of the image:
A fluid is passing through an infinitely long two dimensional pipe, with a hole in it (form L to L+a). Clearly some fluid will be ejected out from that hole.
Now,
...
2
votes
0
answers
125
views
Adiabatic equation of state for a star
I am a little confused. To describe stars the adiabatic equation is often used: P = k$\rho^{\gamma}$. For a monatomic gas, the following holds: $\gamma = \frac{5}{3}$.
However, since the sun is not ...
1
vote
0
answers
44
views
Interpretation of fictitious forces arising from convection in fluid flow in a curvilinear coordinate system
the time rate of change of linear momentum of a fluid with density $\rho$ and velocity vector $\mathbf{u}$ is given by the material derivative:
$$\frac{\mathrm{D} \rho \mathbf{u}}{\mathrm{D}t} = \frac{...
0
votes
1
answer
91
views
Plot Nusselt number between two plates [closed]
I have a free convection system between two walls (hot, and cold). And I have the temperature distribution between them, solved numerically. Now I need to plot the Nusselt number between walls.
This ...
0
votes
1
answer
24
views
Equation for Convective Heat Transfer
In this Wikipedia link: https://en.wikipedia.org/wiki/Convection_(heat_transfer)#Convective_heat_transfer on convective heat transfer, the following equation for convection is provided:
$$\dot{Q}=hA(T-...
0
votes
3
answers
373
views
If you moved your hand continuously through fire, would it feel hot?
If you quickly pass your finger or hand through a candle or fire, it doesn't feel very hot (great party trick). Is this because your hand spends such little time in the fire that there isn't enough ...
0
votes
0
answers
58
views
Can mass transfer occur without a concentration difference? (analogous to the question, can energy transfer occur without a temperature difference?)
The driving force for diffusion of mass is a concentration gradient. However, mass can be transported both in a diffusive and a convective (due to the flow of a fluid) way. By definition, the combined ...
0
votes
1
answer
201
views
Building a house that is warm in winter and cool in summer [closed]
The Northern house architecture is strikingly different from that used in the South (I am not taking extreme cases, thinking more of Northern or Germany versus Mediterranean:
Thick layers of thermal ...
10
votes
3
answers
2k
views
Physics of the convection oven
I don't understand how convection ovens can cook food faster than radiant heat ovens. Guides to convection ovens claim that they cook food 25-30% faster than radiant heat ovens, but do not really ...
-1
votes
3
answers
191
views
Why don't the "trapped warm air" just fly out of the holes in blankets?
I've read a lot of explanations that the holes of a blanket somehow trap air which keeps the air in place.(including this: https://www.reddit.com/r/explainlikeimfive/comments/10lj0h/...
1
vote
1
answer
51
views
Convective derivative N-S
This is probably an easy answer, but I've not been able to find it yet -
Why in some formulations of the N-S equations (for example here https://www.grc.nasa.gov/www/k-12/airplane/nseqs.html), is the $...
1
vote
1
answer
51
views
How does air cool as it nears the poles?
I understand air is heated by the equator causing it to rise towards the poles. But why does air cool and sink after nearing the poles. Shouldn't the air still possess heat, after being heated by the ...
0
votes
1
answer
69
views
Geometrical Rotation and Rotation operator relation and ambiguity
In Cohen-Tannoudji's it is said (vol.1 page 694):
$|\psi \rangle$ is the state of the system before the rotation.
$|\psi ' \rangle$ is the state of the system after the rotation.
$|\psi ' \rangle= R |\...
0
votes
1
answer
71
views
What causes the lamp above my gas stove to swing like a pendulum when the stove is on?
There is lamp hanging above my gas stove, which when the stove is turned on performs pendulum-like oscillations in an oval trajectory, not unlike the dynamics of Lissajous patterns. The lamp is ...
0
votes
0
answers
38
views
The local and average Nusselt number in a square cavity
I am in the process of programming the local & average Nusselt number in a left vertical wall but my Matlab script gives me inappropriate values and it doesn't change with changing of Rayleigh ...
0
votes
0
answers
154
views
Average Nusselt number and heat transfer coefficient
For the flow over a flat plate of length L, how the average Nusselt Number over a length L is equal to
$$Nu_{avg} = \frac{h_{avg}L}{k_f}$$
where $h_{avg}$ is the average heat transfer coefficient over ...
0
votes
0
answers
22
views
Gas to solid heating?
I am doing a study on the phenomenon of pyrolysis. This is purely a study of the underlying physics. Generally, in practice, pyrolysis feedstock is heated conductively but we are examining the energy ...
0
votes
1
answer
39
views
Convection from rapidly evaporating liquid inside vacuum chamber?
If we have a liquid that is is being actively heated and is rapidly evaporating (let's say around 1g/s for Ag at 1500K) inside a vacuum chamber, can convection around the liquid/vapor interface occur ...
4
votes
1
answer
91
views
Small parameter expansion to obtain convective heat transfer solution of the heat equation
I'm trying to follow along with this paper on perturbative transport analysis. The section I'm stuck on analyzes the heat equation in 1D slab geometry. To get a diffusion-only solution is relatively ...
0
votes
1
answer
73
views
Is destratification ventilation more efficient at ceiling level?
This question is more of a practical physics question. In buildings with heating systems, heated air tends to rise and create air temperature stratification - hottest air near the ceiling, coldest air ...
4
votes
5
answers
784
views
Can conduction alone cause thermal equilibrium?
From the little I know about physics and conduction specifically, it seems to be that a system in which the heat transfer is done solely by conduction, cannot reach thermal equilibrium.
I'll preface ...
1
vote
1
answer
362
views
How to treat Nusselt numbers that are smaller than one?
The Nusselt number is defined as
$$\text{Nu}=\frac{h\delta}{k},$$ with
$h$ as convection heat transfer coefficient,$\text{W}/(\text{m}^2\text{K})$,
$\delta$ as characteristic length, $\text{m}$, and
$...
0
votes
2
answers
101
views
How can the rate of convection up a chimney be computed? [closed]
I have various physics textbooks, but none of them explain exactly how to compute the rate of convection.
I assume that Newton's Law of Cooling should be used by I can't figure out how to apply it to ...
0
votes
3
answers
35
views
Heat Transfer Conservation in a natural convective flow
In a initially stationary area, suppose I have a heat source of say 10 W.
Now if I consider a fictional cube (box)/ boundary of say volume 1 m3 around it. And I sum up the heat across the surfaces of ...
1
vote
0
answers
32
views
Spectrum of the background radiation on earth
Suppose we consider an object of temperature $T_o$, whereas the air temperature is $T_a$. The object will exchange heat with its environment via a range of processes:
Radiating and absorbing heat
...
0
votes
1
answer
124
views
Heat Transfer Rate to Surroundings of Heating Battery
I think this is probably a trivial question, but I am trying to understand how to do a thermodynamic calculation for heat transfer. I want to make a basic resistive heater by shorting a battery, but I ...
9
votes
4
answers
970
views
How does hot air rise?
If a balloon is filled with hot air, it is rising due to buoyancy: the mass of the hot air inside the balloon is lower than the mass of the same volume of the cold air outside the balloon cavity.
...
1
vote
0
answers
37
views
Rayleigh Benard Convection analyzed as heat engine?
The treatment I've seen for Benard convection in a textbook is to derive approximate equations of motion and then show that there is a periodic solution matching the boundary conditions. This is ...
1
vote
1
answer
348
views
How much energy is required to keep water hot?
I have a hot-water jug that keeps water hot (say, at about $70\, ^{\circ} C$). To compute its operation cost, I would like to compute its daily energy consumption. The label on the jug says "800 ...
0
votes
2
answers
151
views
How to modify single phase fluid/solid coupled PDEs to account for a phase change?
I have two PDEs that model both fluid and solid temperature change due to fluid flow through a packed bed. Schematic and equations here (where the f and s subscripts are for the fluid and solid ...
0
votes
0
answers
97
views
How to calculate convection between a hot and a cold area through a circular hole?
Consider the below problem:
where we have two half infinite worlds with $T_2 < T_1$ and a circular hole with the diameter of $D$ in an infinite isolating wall. There is no ...
0
votes
2
answers
121
views
Calculate how hot air will get passing source of heat
Is there a way to calculate how warm air will get when it passes a source of heat knowing how fast the air is moving?
For example, let's say there is an object being heated by 40W of electricity and ...
1
vote
1
answer
124
views
Understanding Convectional Rainfall
Consider the following explanation of convectional rainfall given in a VII grade textbook.
When air comes in contact with the hot surface of the earth, it gets heated, becomes light and rises in the ...
0
votes
1
answer
95
views
In this scenario, if I continue to increase the length of heat exchanger how will the temperatures change?
Consider a counter flow double pipe heat exchanger inside which the cold fluid enters at 20 degrees Celsius and hot fluid enters at 100 degrees Celsius. Initially the length of the heat exchanger is ...