Conduction usually occurs in solids, through molecular collision. Convection occurs in fluids by mass motion of molecules in the same direction. In contrast, Radiation takes place through the vacuum of space and does not heat up the intervening medium. The particles in solids are tightly packed and can only vibrate. The particles in liquids also vibrate but are able to move around by rolling over each other and sliding around. In gases, the particles move freely with rapid, random motion, so the speed of conduction and convection is slower than radiation.

It depends on the properties of materials that used for heat transfer by conduction and the temperature difference between two solid bodies.

For free convection, the properties of the fluid and material, in addition to the value of heat flux or the temperature difference between the fluid and the solid material.

For forced convection, the same parameter in free convection and also flow velocity.

for radiation heat transfer it depends on the temperature difference and the reflective and absorbent and transitivity.

- Conduction is diffusion equation with instantaneous "feel" in all of the domain or mathematically speed of infinity.

- Convection is advection so the speed is the speed of gas or fluid phase which carries the heat.

- Radiation is speed of light

Interesting question. My 2 cents Worth of information:

1) In thermal conduction, Fourier's Law (parabolic formulation of heat conduction) presupposes an infinite speed of heat proagation. This is obviously not true, and several modifications are proposed, which attempt to include a finite propagation speed (which leads to a hyperbolic form of the conduction Equation). The propagation speed varies, depending on material and phase, but has been shown to be of the same order as the speed of sound in the material. In practice, this means that the term describing the temperature propagation speed is normally insignificant (read: does not limit the temperature propagation), and that the conventional Fourier's Law formulation can be used.

2) Convection: Same as conduction in the direction perpendicular to the flow; in the flow direction it is normally the flow velocity.

3) Radiation: Speed of light (photons)

1) By conduction: Heat moves from one person to another through the material without any transfer of the latter. Conduction therefore occurs in solids, it corresponds to the propagation of the kinetic energy of thermal agitation possessed by the particles constituting the material.

2) By convection: in a fluid, temperature differences produce density differences that can lead to movements of the material, known as convection movements. We have material transfer, that's why we don't find this means in solids.

3) By radiation: Bodies emit energy through their surfaces in the form of radiation. It is a means that does not require material support, so it can be found in a vacuum. All transparent bodies allow heat to spread like this.

The speed of heat transfer by conduction and convection is slower than radiation.

Dear Dr. Naess,

A magnificent answer, which worth indeed more than two cents. Can you please provide a reference of your point (1)?

Thanks.

I think that's depends on the conditions of cases that u r studied

Dear Dr. Garcia,

A nice article on fundamentals of heat propagation may be read found here

https://en.wikipedia.org/wiki/Relativistic_heat_conduction

The 'speed of heat propagation', C=sqrt(Alfa/Thau), where Alfa is the thermal diffusivity, and Thau is the relaxation time. According to e.g. S. Rastegar: "Hyperbolic heat conduction in pulsed laser irradiation of tissue" (Proc. SPIE, Aug 8, 1989, doi10.1117/12.951954) the relaxation time of gases (Nitrogen) is about 10^(-9) s and for metals about 10^(-11) s. (They referred to Luikov: "Analytical Heat Diffusion Theory" Academic Press, 1968 as the source for relaxation times.)

Dear Dr. Garcia

In solids, heat transfer by heat conduction from molecular to another. In liquids, heat transfer by heat conduction and heat convection in the same time. In Gas, heat transfer by heat radiation only. Moreover, the pressure in gas larger than liquids and solids respectively. The pressure proportional with speed, then the conclusion is the heat speed in radiation larger than speed in liquid than in solids.

Dr. Mohammadein , please check the answer of Dr. Naess, which is flawless.

For anyone interested on finite heat propagation speed I recommend the reference Joseph D.D. & Preziosi L., "Heat Waves", Reviews of Modern Physics, Vol. 61, no. 1, pages 41-73, January 1989.

Basic modes of heat transfer are conduction, convection and radiation. The mechanism of heat transfer is different for three different modes. In conduction, the transfer of heat will be from one body to other body or one body to any physical contact and there will be no movement of molecules. In convection, the transfer of heat will be in a fluid medium where molecules move along the movement of fluid and transfer through medium. In radiation, the transfer of heat will be from one body to other body with no medium and travels through space. Conduction is dominant in solids, Convection is dominant in liquids and radiation is dominant in gases. Hence, in order to arrive speed (modes of heat transfer different) for three modes of heat transfer, it depends for the purpose and constraints it was calculated and in general cannot be concluded.

Thank you.

If you take any body (solid, liquid or gas) at a given state and at a given moment, each of its points (elementary particles) is characterized by a temperature which is not necessarily equal to the temperature of the neighbor. This is the case following a heating or a cooling. It will therefore be submitted to a thermal agitation (collision) during which these particles move while carrying heat and electricity until a state of "equilibrium" characterized by a mean kinetic energy. This phenomenon called conduction or molecular diffusion is microscopic and is not characterized by a velocity in the sens of Newtonian macroscopic physics; This is why this phenomenon is characterized by its thermal diffusivity (m2 / s) and not by a speed (m / s). It should be remembered that this phenomenon exists in solids, liquids and also gases (even at low pressure)

As the conduction transfer continues, the temperature of each point (volume element) will increase until a flow can be observed ( example: ascending movement following bottom heating: Natural Convection). The threshold temperature corresponds to a Rayleigh number around 1704 for a pure fluid. In the case of a solid one can reach a phase change : the fusion

This movement of molecules contained in an elementary volume is visible and therefore following it in time one can measure the velocity field. The heat is now transported by the flow of the fluid and this mode is called thermal convection.We can distinguish two types of convection:

Natural: where the heat is transported by the flow of fluid due to the density gradients and the gravity ( ARCHIMEDE );

- Forced: the flow is created artificially ex: by a fan

Each of these convections may be laminar or turbulent.

Earth Weather and especially the climate change of our planet is managed mainly by the convection movement.

In nature, this heat is generated mainly by the solar radiation that reaches the surface of our earth. The thermo-nuclear fusion inside the sun produces a surface temperature of 6000 ° K. This heat is transferred to the outer limit of the terrestrial atmosphere "without" loss in the "void" separating it from the sun. This phenomenon of exchange between two bodies did not require any intermediate medium and no movement, unlike the previous 2 modes of exchange, this is called thermal radiation. This type of radiation is part of the broader electromagnetic radiation. This thermal radiation arrives in the form of a spectrum of several waves which correspond, among other things, to the light whose velocity of propagation in vacuum is well known.

But as soon as this radiation enters the the Earth's atmosphere, it will be partially absorbed by clouds, aerosols and others; So just a part will get to the surface of the earth. Some of which will be radiated this time by the earth to the sky after transforming the initial spectrum to emit essentially in Infra-red.This will regulate the temperature of the earth . But as soon as there is Co2, water vapor, Nox, etc., in our atmosphere, these gases will reflect and emit some of this radiation towards the earth causing its temperature to increase: 'greenhouse effect