Questions tagged [convection]

transport mechanism involving bulk motion of a medium

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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 ...
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-1 votes
3 answers
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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/...
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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 $...
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How to calculate the flow rate of a convection current?

If a convection current is streamlined (no turbulent flow) and you know the heat energy input, how would you calculate the flow rate? If it's not possible to calculate with just the heat energy input, ...
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How much heat energy would translate into the kinetic energy of a convection current in water if the flow is streamlined?

Assuming that we have a mechanism by which a convection current can be perfectly streamlined, how much of the heat energy absorbed by the water would translate into the kinetic energy of the ...
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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 ...
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Understanding thermal models of cavitation

Cavitation is a phenomena that occurs in liquids when the local static pressure drops below the vapor pressure; bubbles form within the liquid. I am studying cavitation in incompressible, two-phase (...
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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 |\...
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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 ...
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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 ...
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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 ...
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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 ...
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1 answer
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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 ...
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2 votes
0 answers
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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 ...
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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 ...
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5 answers
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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 ...
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1 vote
1 answer
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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 $...
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Where should a cold carton of milk be placed in a styrofoam picnic box to keep it cold/cool for the longest time?

If you have a cold carton of milk that you place on the floor of an otherwise empty (apart from air) styrofoam picnic box (and replace the lid after) in a room at room temperature, should you place it ...
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2 answers
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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 ...
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Isn't the Newton's cooling law for heat convection too macroscopic for CFD simulations?

In Computational Fluid Dynamics simulations, the space is discretised into a huge number of tiny cells and, therefore, one has the opportunity to use a model of heat transfer that takes into account ...
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3 answers
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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 ...
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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 ...
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1 answer
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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 ...
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6 votes
4 answers
357 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. ...
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What is the relationship between air temperature and updraft velocity?

So I was playing Zelda Breath Of The Wild recently. And in the game, you can set grass on fire to create an updraft. (see the gif below) Which led me to ask myself : "Wait, how hot are these ...
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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 ...
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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 ...
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2 answers
125 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 ...
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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 ...
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2 answers
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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 ...
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Vortex methods and Helmholtz theorem

I'm pretty new to vortex methods in fluid dynamics, I found in this paper (page 12, line2) that Helmholtz theorem states: that the dynamics of the vorticity field can be described by vortex elements, ...
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Maximum rate of heat transfer in a balanced counter flow heat exchanger in NTU method

In effectiveness NTU method for heat exchangers, we have effectiveness of the heat exchanger defined as $$\epsilon = \frac{Q_{actual}}{Q_{max}}$$ Where $$Q_{max} $$ is the maximum rate of heat ...
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1 answer
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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 ...
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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 ...
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Maximum heat transfer rate in effectiveness NTU method for heat exchangers

The effectiveness NTU method is used for determining the heat transfer rate for a heat exchanger, with known inlet temperatures and surface area. It makes use of effectiveness which is defined as $$\...
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1 answer
202 views

Arithmetic and Log mean temperature difference of a double pipe parallel flow heat exchanger

Consider a double pipe parallel flow heat exchanger. For such a heat exchanger the LMTD is given as $$\Delta T _{LM}= \frac{\Delta T_i-\Delta T_e}{ln(\frac{\Delta T_i}{\Delta T_e})} $$ The relation ...
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2 votes
2 answers
91 views

What does this assumption mean in heat exchanger analysis?

I'm studying heat transfer at graduate level, and in the process of deriving LMTD of a heat exchanger, the book took the following assumption : Any axial heat conduction will be neglected. What that ...
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2 answers
45 views

'Convective' Fourier number

I'm working with a numerical solution to a 1D advection-diffusion equation for a multiphase problem, with convective exchange between the gas and solid phase. A non-dimensional parameter which comes ...
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1 answer
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Entropy Change in Radiation and Convection

When a system loses it's internal thermal energy by emitting radiation or by convection what is change of the systems entropy mathematically? I saw this question's answers for radiation only but still ...
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0 answers
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Is this value of Critical Reynolds Number also valid for non circular ducts?

The flow remains laminar in case of internal flow through pipes up to a Reynolds number of 2300, for most practical cases. Is this value valid only for circular pipes or even for non circular ducts?
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2 answers
456 views

Does the convective heat transfer coefficient depend on temperature difference?

Consider a flat plate at a higher temperature than the fluid which is flowing over it, with some velocity. Forced convection is taking place. I'm convinced that the value of heat transfer coefficient ...
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31 votes
8 answers
10k views

If air is a bad conductor, how does fire heat up a room?

If air is a bad heat conductor, how does fire heat up a room? Could someone help me, as I really don't get this?
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1 vote
1 answer
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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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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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1 vote
1 answer
72 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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1 answer
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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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1 answer
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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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0 votes
1 answer
179 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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1 vote
1 answer
56 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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1 answer
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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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