Questions tagged [convection]

transport mechanism involving bulk motion of a medium

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299 views

Which are the best books I can refer for turbulent heat transfer and its computation?

I am a graduate student. I wanted to do computation on heat transfer problem mainly heat convection. I am working on turbulent regime. Can you kindly suggest some good books so that I can get ...
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52 views

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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A theoretical model for evaporation occurring when wind is blowing over the surface of water

I've recently done an experiment in which I was studying the variation of the cooling rate of hot water in a draft (wind). The air was blown directly over the surface of the water (kept in a ...
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300 views

Natural convection and buoyancy: What is a better description of $g$?

In their derivation of the Grashof number (Gr), Çengel and Ghajar make the following comment: Note that there is no noticeable gravity in space, and thus there can be no natural convection heat ...
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597 views

Evaporation rate of water with forced convection

A steel pot (diameter 200 mm) is fully filled with water (2 kg of water) and the water is maintained at a constant temperature of 40°C by supplying heat from an electrical tabletop burner. Assume ...
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2k views

Source for Nusselt number correlation for cooling sphere under forced convection

I've been trying to find a Nusselt number correlation for a sphere cooling in a forced gas cross flow where the sphere temperature is much higher than the free stream temperature. I want something ...
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32k views

Is heat from a stovetop, transfered through convection, radiation or conduction?

It doesn't appear to be convection, as there are no moving objects (or are there); probably not radiation, so it is conduction? I am confused because it seems that the molecules in the stove does not ...
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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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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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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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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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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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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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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
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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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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
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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4answers
14k views

Realistic calculation of heat loss for pipe

Good day everyone, I am new on this site and I hope to find here help, since I am not going anywhere with the literature I have found. I try to calculate realistically the heat loss of a hot, ...
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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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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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How does smoke move in the air and how can one direct it?

Let us assume there is a closed room with two people in it and only one of them is smoking a cigarette. Which equation describes the behavior of the smoke from the cigarette spreading? Is it “...
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Why does a match go out if you hold it so the flame is up?

I have noticed something which may have been noticed already: take a match, and light it, then hold it so that the flame is directly above your hand. The flame will eventually go out. My guess as to ...
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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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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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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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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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1answer
2k views

What causes the differential rotation of the Sun?

I have read the explanation in Wikipedia, but the problem is that because I'm a beginner in physics, I don't quite understand what more than half of the words mean! Here is the explanation from ...
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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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What is the exact difference between diffusion, convection and advection?

I have tried to explore the information but still not very clear on the exact difference between diffusion, convection and advection. Can anyone help me out to clear my concept?
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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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1answer
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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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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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8k views

In what situations do I use the characteristic length of a fin to find the surface area?

So I'm learning about fins in heat transfer and it seems that there are two separate formulas for the surface area of a rectangular fin of length L, width w and thickness t. The fin is attached to a ...
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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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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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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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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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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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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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Why are radiators always placed under windows?

I don't know if anyone else has noticed this but in most buildings and most rooms, radiators are predominantly placed under a window. Now, in my eyes, that is the worst place to put them; hot air ...
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1answer
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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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463 views

What is meant by “stable/unstable to convection” in the context of the solar atmosphere?

In studying the Schwarzschild stability criterion (for the solar atmosphere); $$\left\lvert\frac{dP}{dr} \right\rvert\lt\frac{\gamma P}{\rho}\left\lvert\frac{d\rho}{dr}\right\rvert$$ From my notes are ...
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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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1answer
302 views

Convective heat transfer

At y = 0 It is said that the heat is transferred by conduction because the fluid has a velocity equal to 0 (no-slip conditions). But the temperature of the fluid for y=0 is equal to Ts(temperature of ...