What is Convection: Definition and 194 Discussions
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convection is unspecified, convection due to the effects of thermal expansion and buoyancy can be assumed. Convection may also take place in soft solids or mixtures where particles can flow.
Convective flow may be transient (such as when a multiphase mixture of oil and water separates) or steady state (see Convection cell). The convection may be due to gravitational, electromagnetic or fictitious body forces. Heat transfer by natural convection plays a role in the structure of Earth's atmosphere, its oceans, and its mantle. Discrete convective cells in the atmosphere can be identified by clouds, with stronger convection resulting in thunderstorms. Natural convection also plays a role in stellar physics. Convection is often categorised or described by the main effect causing the convective flow, e.g. Thermal convection.
Convection cannot take place in most solids because neither bulk current flows nor significant diffusion of matter can take place.
Hi,
I have run into a problem and can't find an answer. I am trying to estimate the heat transfer coefficient for water INSIDE a vertical cylinder exposed to uneven heating.
The cylinder is 1m high with a diameter of 0.035m. D/L is then 0.035.
I have calculated the Grashof number 417200 and...
Convection heat transfer rate decided by Newton's cooling law doesn't have a relative velocity between the fluid and the solid surface term. But intuitively doesn't blowing faster on a hot surface make it cool quicker?
Hello, I have an interesting question about water natural convection without an external heat source.
Say we have a high tower filled with water. The tower walls don’t allow any heat to escape or enter the system. There is a constant gravitational force present in a system (g-force). At the...
I'm looking for the analytical solution for the 1D convection diffusion equation with a constant heat flux.
Boundary conditions:
The domain I'm looking at is x from 0 meters to 1 meter. The temperature at x=0 is T=0 degrees Celsius. At x=1, T=100 C.
I'm given the equation...
Hello
Wondering if anyone can help me towards finding an equation to calculate the heat transfer due to forced convection? Can't seem to find a good source for one at all, needs to be referenced for my MEng project.
Currently have for natural convection:
Q = h A (ΔT)
Thank you...
Hello everyone,
So I am an Aero-Thermo Intern at Pratt and Whitney, and my supervisor gave me the following problem to set up a mathematical Excel model for the temperature gradient inside an enclosed box with a heat source underneath it to be used in thermal analysis of an engine component...
I am trying to develop a spreadsheet to calculate a convection coefficient with altitude as one of the inputs. What variables should I consider? The most obvious are pressure and temperature, but I'm sure there is more to it than that.
Homework Statement
A silicon chip measuring 5mm by 6mm and of thickness 1mm is embedded in a ceramic substrate. At a steady state, the chip has an electrical power input of 0.3W. The top surface of the chip is exposed to a coolant whose temperature is 20°C. The heat transfer coefficient for...
Homework Statement
Solve u_t -k u_xx +V u_x=0
With the initial condition, u(x,0)=f(x)
Use the transformation y=x-Vt
Homework Equations
The solution to the equation u_t - k u_xx=0 with the initial condition is
u(x,t)=1/Sqrt[4\pi kt] \int e^(-(x-y)^2 /4kt)f(y) dy
The Attempt at a...
Hello, PhysicsForums!
I've been discussing what little I know of plate tectonics with someone and the other party has suggested that convection cells within the mantle (assuming convection really occurs at all) should eventually move such that they no longer push more matter up through...
I need some clarification regarding the following formula:
Qconv = Pn x Qs x b x ϕ
this equation is used to determine the cooking appliance's convective heat output in kw, 'b' and 'ϕ' are coefficients easily determined. I am confused whether to use the actual capacity of the appliance for...
Hi
I'm trying to workout the velocity of air inside a pipe that's sealed and ten meters long.
I've found this Air Velocity
vc = 0.65 [g l dt / (273 + te)]1/2 (1)
where
vc = velocity in center of airflow (m/s)
g = 9.81 - acceleration of gravity (m/s2)
l = vertical distance from bottom of...
Homework Statement
Consider a flat plate subject to parrallel flow (top and bottom) characterized by u∞ = 5 m/s and T∞ = 20°C.
Determine the average convection heat transfer coefficient and convective heat transfer rate with an L = 2m long and w = 2m wide flat plate for airflow and...
Dear all
I got a value of 37 for h ( convection heat transfer )
Does it consider as free or natural convection heat transfer coefficient?
Thanks in advance !
And Sorry its not a Homework problem :)
I am confused as to how heat transfer really works. I've read all about conduction/convection/radiation. Radiation makes sense to me, electrons enter a higher band, come back down and give off energy in the form of EM waves. These waves can then be absorbed by other atoms causing electrons to...
Hello everyone,
I am trying to model the flow of steam through a vertical cylinder. Ultimately I would like to find the surface temperature as a function of the length of the pipe. I am assuming the steam cools as it moves upward along the length of the pipe and it is a steady-state process...
Hi all,
I have a unique problem that I need some direction on. I have a .5x.5x.75 meter adiabatic chamber. No heat loss from chamber walls to the atmosphere.
Inside the chamber there is an electrical flat heater plate .5x.05 meter with constant 100W. A fan blows air over the heater bar at...
I used to think that the heat does rise even in solid metals with no gas/liquid around
(No density argument is possible then.)
But couldn't find anything describing or even verifying it.
I'am pretty sure the gravitationally induced anharonicity in the atomic core potentials
should have at...
In my book I'm reading about heat, specifically convection. There is an image of a person with a drink in their hand, suntanning while a fan blows on them. I get that the drink represents conduction, the sunlight is radiation, but how is the fan convection?
I have a question...
Suppose you have a thin and long rectangular piece of aluminum.
You heat one end (pt A). The thermal resistance (steady state) from one end (pt A) to the other (pt B) would be
θ=ΔT/W
Where W= Watts of heat energy.
ΔT = temperature of ptA - temperature of ptB...
Homework Statement
A thin rod of length ∏ is heated at one end to temperature T_0. It is insulated along its length and cooled at the other end by convection in a fluid of temperature T_f . Find the transient and steady-state temperature distribution in the rod, assuming unit thermal diff...
Ok, I have some questions to answer for this lab report, and one of them has left me clueless. The experiment was as follows:
An apparatus is made of a rectangular circuit (of known dimensions) of tubes filled with water with thermometers visible at various points throughout the circuit. No...
hi,
if i have a cylindrical rod of copper and i heat is at one end face while the other surface areas are open to the atmosphere then besides the conductivity of copper i will also need the convective heat transfer coefficient to analyse the setup.
my question is : how do can we estimate...
Homework Statement
In this flat plate heat exchanger, rippled plates (0.3 m thick each) create channels that allow heat transfer between a hot and a cold fluid. Conduction occurs then from the heat temperature difference between the two fluids while convection occurs as the fluids move...
I need to develop a simple model in Comsol, but I have no experience with it. I need to see what the convective currents and velocity fields are at certain temperatures in a glass optical fiber. I have the information about the fiber and its dimensions. The fiber is going to be at around its...
I have a heating element. This heating element is placed on a wall. If I want the heat to be supplied horizontally would a relflector blocking the top of the heating element work?
to prove this are these two statements below correct??
1. Heat rises so by blocking the top of the heating...
Hey,
I was having this very silly problem in heat transfer. Please could you see what's going wrong !
I am working on defrost water tray modelling. In that, the water collected during defrost is heated by the compressor o/p coil so as to aid evaporation. I was trying to find out the heat...
The problem is as follows:
\frac{\partial u}{\partial t}=k\frac{\partial^{2}u}{\partial x^{2}}+c\frac{\partial u}{\partial x},
-\infty<x<\infty
u(x,0)=f(x)
Fourier Transform is defined as:
F(\omega)=\frac{1}{2\pi}\int_{-\infty}^{\infty}f(x)e^{i\omega x}dx
So, I took the Fourier...
Homework Statement
Want to heat an object of given length, 1cm, from both sides by convection. It is at a uniform temperature (5 degrees C) and is to be heated in an oven (200 degrees C) from both sides until its center reaches 100 degrees C. (the density of the material, its convection...
I'm learning c++ and currently trying to write a little programme for 1D linear convection (wave equation).
I have managed set all boundary and initial conditions as well as a mesh.
But I have reached the point where I can't understand why the program does what it does.
In red I...
I'm studying the effect of a magnetic field on a non-conducting fluid, such as liquid oxygen or liquid hydrogen. As I know a convective flow can be driven, if the extern magnetic field is strong enough. The extern magnetic field can be uniform or nonuniform, but the effect of convection is much...
Hi,
I've did an experiment today, a forced convection in circular tube where the heat flux is constant along the length.
my result seems not follow what it should be..
[PLAIN]http://img688.imageshack.us/img688/8015/expresult.png
what I'm wondering is shouldn't it in a fully...
Homework Statement
Air at 20kPa and 20°C flows across 5cm diameter cylinder at velocity of 20m/s. Compute the drag force exerted on the cylinderHomework Equations
the formula should be use is this, FD = CD*A*den*V^2 / 2
where FD & CD is the drag force and drag coefficient, respectively
The...
Hi All,
I'm trying to understand heat so I'm going to say what I understand and ask you to please tell me if I am correct. Thank you.
Radiative Heat - This is heat transferred in the form of electromagnetic radiation and does not need a medium. If in a vacuum, the radiation will travel...
Homework Statement
Heat transfer due to natural convection in a rectangular enclosure with vertical walls that are at different temperatures.
Need a correlation for Nusselt number that could handle an aspect ratio A (width/height-ratio), A > 100 and Rayleigh number Ra > 10³. Also, Prandtl...
Hi all,
I have recently observed a very strange thing (or rather, I shall say unexpected phenomenon). When I deposited some charged micro-particles between 2 electrodes (vertical plates) with a DC bias in hexadecane oil, I expected the negatively charged particles to go to the positive...
Homework Statement
A vertical steam pipe of outside diameter 7.5 cm and height 4 m has its outer surface at the constant temperature of 950C. The surrounding air is at atmospheric pressure and at 200C.
How much heat is delivered to the air by natural convection in 1 hr?
Homework Equations...
Hello!
I'm currently reading papers about ferrofluids and trying to understand the physics behind this topic.
I do understand, that if we consider a vertikal temperature gradient, we get a antiparallel magnetization gradient. The ferrofluid with a the higher magnetization will move to the...
Hello, everyone!
I could not imagine what force makes this convection under concentration gradients , no matter how hard I thought.
Maybe we can use principle of entropy increase to explain it, but that is not vivid to me.
anyone can help me? Thans!
Hello,
I am having some problems with COMSOL multiphysics 3.5a. I am using 2D weakly compressible flow with general heat transfer and am trying to model a natural convection loop.
The Approach I have taken is to use properties of the material for a certain range of temperatures to...
Hi everyone,
I could use some help to validate (or invalidate) and improve my equations as I am trying to model 2D heat dispersion in a Spa.
Basicaly my model consist of some air inside a cabinet, the cabinet is framed by 2 different insulants each insulant has heat flux on boundaries (a...
Hey I've got this question and the answer that is giving is 219.12 but i keep getting 319 could someone please help me out here's the question.
Consider a person standing in a room. The room air temperature is
maintained at 22OC at all times. The inner surface of the walls, floors and
the...
Hi, I have a pseudocode I would like to try and implement and understand the programming of it in python. The physics and maths of the case is no problem, but the implementation of the code in Python is something I'm not familiar with.
The problem is a simple 1D linear convection using...
Homework Statement
Two large thin aluminum plates, the first polished (emittance = 0.05 and the second painted black, are placed horizontally outdoors, where they are cooled by air at 283K. The heat transfer coefficient is 5 W/m^2-K on both the top and bottom. The top is irradiated with 750...
Nusselt number:
1) The only correlation I could find so far is the one described here http://en.wikipedia.org/wiki/Natural_convection which seems to be from Churchill/Thelen. There seem however to exist some questions regarding the characteristic length and the value to use for Nuo (see...