# Asymmetric heat conduction and negative di

Okay, it is finally time to completely solve a partial differential equation. In the previous section we applied separation of variables to several partial differential equations and reduced the problem down to needing to solve two ordinary differential equations.

In this section we will now solve those ordinary differential equations and use the results to get a solution to the partial differential equation. We are going to do the work in a couple of steps so we can take our time and see how everything works. The first thing that we need to do is find a solution that will satisfy the partial differential equation and the boundary conditions.

At this point we will not worry about the initial condition. Okay the first thing we technically need to do here is apply separation of variables. This leaves us with two ordinary differential equations. We did all of this in Example 1 of the previous section and the two ordinary differential equations are.

The positive eigenvalues and their corresponding eigenfunctions of this boundary value problem are then. Note however that we have in fact found infinitely many solutions since there are infinitely many solutions i. So, there we have it. The function above will satisfy the heat equation and the boundary condition of zero temperature on the ends of the bar.

The problem with this solution is that it simply will not satisfy almost every possible initial condition we could possibly want to use. This is actually easier than it looks like. This is almost as simple as the first part. Doing this gives. The Principle of Superposition is, of course, not restricted to only two solutions. For instance, the following is also a solution to the partial differential equation. Doing this our solution now becomes.Newton's law of cooling states that the rate of heat loss of a body is directly proportional to the difference in the temperatures between the body and its surroundings.

The law is frequently qualified to include the condition that the temperature difference is small and the nature of heat transfer mechanism remains the same.

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As such, it is equivalent to a statement that the heat transfer coefficientwhich mediates between heat losses and temperature differences, is a constant. This condition is generally met in heat conduction where it is guaranteed by Fourier's law as the thermal conductivity of most materials is only weakly dependent on temperature.

In convective heat transferNewton's Law is followed for forced air or pumped fluid cooling, where the properties of the fluid do not vary strongly with temperature, but it is only approximately true for buoyancy-driven convection, where the velocity of the flow increases with temperature difference. Finally, in the case of heat transfer by thermal radiationNewton's law of cooling holds only for very small temperature differences, and a more accurate description is given by Planck's Law.

Sir Isaac Newton did not originally state his law in the above form inwhen it was originally formulated. Rather, using today's terms, Newton noted after some mathematical manipulation that the rate of temperature change of a body is proportional to the difference in temperatures between the body and its surroundings.

This final simplest version of the law given by Newton himself, was partly due to confusion in Newton's time between the concepts of heat and temperature, which would not be fully disentangled until much later. When stated in terms of temperature differences, Newton's law with several further simplifying assumptions, such as a low Biot number and a temperature-independent heat capacity results in a simple differential equation expressing temperature-difference as a function of time.

The solution to that equation describes an exponential decrease of temperature-difference over time. This characteristic decay of the temperature-difference is also associated with Newton's law of cooling. Convection cooling is sometimes said to be governed by "Newton's law of cooling. When the heat transfer coefficient is independent, or relatively independent, of the temperature difference between object and environment, Newton's law is followed. This independence is sometimes the case, but is not generally so. The law holds well for forced air and pumped liquid cooling, where the fluid velocity does not rise with increasing temperature difference.

Newton's law is most closely obeyed in purely conduction-type cooling. However, the heat transfer coefficient is a function of the temperature difference in natural convective buoyancy driven heat transfer.

In that case, Newton's law only approximates the result when the temperature difference is relatively small. Newton himself realized this limitation.

A correction to Newton's law concerning convection for larger temperature differentials by including an exponent, was made in by Dulong and Petit. Another situation which also does not obey Newton's law, is radiative heat transferbeing better described by Stefan-Boltzmann law as varying with the 4th power of absolute temperature. The statement of Newton's law used in the heat transfer literature puts into mathematics the idea that the rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings.

For a temperature-independent heat transfer coefficient, the statement is:. The heat transfer coefficient h depends upon physical properties of the fluid and the physical situation in which convection occurs. Therefore, a single usable heat transfer coefficient one that does not vary significantly across the temperature-difference ranges covered during cooling and heating must be derived or found experimentally for every system that is to be analyzed. Formulas and correlations are available in many references to calculate heat transfer coefficients for typical configurations and fluids. For laminar flows, the heat transfer coefficient is usually smaller than in turbulent flows because turbulent flows have strong mixing within the boundary layer on the heat transfer surface. The physical significance of Biot number can be understood by imagining the heat flow from a hot metal sphere suddenly immersed in a pool to the surrounding fluid.

The heat flow experiences two resistances: the first outside the surface of the sphere, and the second within the solid metal which is influenced by both the size and composition of the sphere.

The ratio of these resistances is the dimensionless Biot number. For systems where it is much less than one, the interior of the sphere may be presumed always to have the same temperature, although this temperature may be changing, as heat passes into the sphere from the surface.

The equation to describe this change in relatively uniform temperature inside the object, is the simple exponential one described in Newton's law of cooling expressed in terms of temperature difference see below.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It only takes a minute to sign up. Namely, because of radiative conduction, reflective aluminum surface and absorbing paint surface, will heat conduction to the left of the drawing be higher than heat conduction to the right?

Is it at all theoretically possible to construct material with such properties in the general case? The second law of thermodynamics forbids materials that conduct better in one forward direction than the reverse direction - such a material placed between two containers at thermal equilibrium would drive the temperature away from equilibrium, decreasing the entropy of the whole system and paving the way for a perpetuum mobile Reflectance and absorbance of material is the same at a given wavelength which is why left-right and right-left processes will happen at the same rate.

You drawing will have anisotropy in the vertical versus horizontal directions - but not left-right versus right-left asymmetry. If we assume that both the aluminum foil and the paint have scalar heat conductivities then their composite will also be scalar, and thus symmetrical. Being both material either poly-crystalline or amorphous this is probably reasonable assumption. Floris suggested that I expand on this, but not being my area I can only summarize a few ideas from Truesdell's Rational Thermodynamics, chapter 7.

Notice that in the 1st law only the symmetric part of the tensor shows up, hence the natural inclination to ignore the skew part. Out of the 32 optical crystal classes 19 are forced to have symmetric conductivity tensor.

In the ensuing decades may experiments were conducted to verify it either theoretically or experimentally Voigt, Curie, Soret, etc. It's tempting to think that heat will flow more readily across a cavity from a high emissivity surface to a low emissivity surface than in the opposite direction when the surface temperatures are interchanged, but this is fallacious.

The reason is that repeated inter-reflection between the surfaces restores symmetry. Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. Asymmetric heat conduction? Ask Question. Asked 6 years ago. Active 6 years ago. Viewed times. Now I wonder, will such a multi-layer material have asymmetric heat conduction properties? Active Oldest Votes.Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter.

All matter with a temperature greater than absolute zero emits thermal radiation. Particle motion results in charge-acceleration or dipole oscillation which produces electromagnetic radiation. Infrared radiation emitted by animals detectable with an infrared camera and cosmic microwave background radiation are examples of thermal radiation. If a radiation object meets the physical characteristics of a black body in thermodynamic equilibriumthe radiation is called blackbody radiation.

Wien's displacement law determines the most likely frequency of the emitted radiation, and the Stefan—Boltzmann law gives the radiant intensity. Thermal radiation is also one of the fundamental mechanisms of heat transfer.

Thermal radiation is the emission of electromagnetic waves from all matter that has a temperature greater than absolute zero. Thermal energy is the kinetic energy of random movements of atoms and molecules in matter.

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All matter with a nonzero temperature is composed of particles with kinetic energy. These atoms and molecules are composed of charged particles, i. The kinetic interactions among matter particles result in charge acceleration and dipole oscillation. This results in the electrodynamic generation of coupled electric and magnetic fields, resulting in the emission of photonsradiating energy away from the body.

Electromagnetic radiation, including visible light, will propagate indefinitely in vacuum. The characteristics of thermal radiation depend on various properties of the surface from which it is emanating, including its temperature, its spectral emissivityas expressed by Kirchhoff's law. If the radiating body and its surface are in thermodynamic equilibrium and the surface has perfect absorptivity at all wavelengths, it is characterized as a black body.

A black body is also a perfect emitter. The radiation of such perfect emitters is called black-body radiation. The ratio of any body's emission relative to that of a black body is the body's emissivityso that a black body has an emissivity of unity i.

Thermal Conductivity, Stefan Boltzmann Law, Heat Transfer, Conduction, Convecton, Radiation, Physics

Absorptivity, reflectivityand emissivity of all bodies are dependent on the wavelength of the radiation.

Due to reciprocityabsorptivity and emissivity for any particular wavelength are equal — a good absorber is necessarily a good emitter, and a poor absorber is a poor emitter. The temperature determines the wavelength distribution of the electromagnetic radiation.

For example, the white paint in the diagram to the right is highly reflective to visible light reflectivity about 0. Thus, to thermal radiation it appears black.SWEP uses cookies to make your visit to our web pages as pleasant as possible.

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By using our services, we assume that you agree to the use of cookies. Further information on data protection can be found in our privacy policy. The most basic rule of heat transfer is that heat always flows from a warmer medium to a colder medium. Heat exchangers are devices to facilitate this heat transfer with the highest possible efficiency.

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A good heat exchanger is able to transfer energy heat from the hot side to the cold side with small thermal losses and high efficiency. The energy heat rejected from the warm medium is equal to the heat absorbed by the cold medium plus losses to the surroundings.

In single-phase heat exchange, there is no phase change in the media. The most common single-phase applications for BPHEs are water-to-water and oil-to-water. The main purpose of the BPHE in oil applications is to cool engine oils, hydraulic oils, transmission oils, compressor oils, etc.

In two-phase heat exchange, there is a phase change on the cold side, the warm side or both. What happens when a liquid or gas changes phase is described below. If heat is added to a liquid, the temperature of the liquid will rise until it reaches its boiling point. Adding further heat will not raise the temperature.

Instead, it increases the gas content of the liquid, resulting in a two-phase mixture of liquid and gas.

The gas generated forms bubbles during boiling. The temperature will not rise until all liquid has been vaporized. When the temperature of the gas becomes higher than the boiling point, the gas is described as superheated. This process is typical of what happens inside an evaporator in a cooling system. The refrigerant enters the evaporator as a liquid and leaves as superheatedvapor. The opposite occurs in a condenser. First, superheated gas is cooled until it reaches its saturation point, where liquid droplets are formed.

When all the gas has been transformed to liquid, the bubble point is reached. Maintaining the same pressure in the vessel while further cooling the liquid leads to a lower temperature, the result being described as a subcooled liquid.

The heat added or lost when the temperature changes without a phase change is called the sensible heatwhile the heat added or lost during a phase change is called the latent heat. The latent heat of the phase transition between liquid and gas is many times higher than the sensible heat of the liquid phase. The ability to transfer heat in liquid and gaseous media depends on the turbulence of the medium.

High turbulence is desirable for efficient transfer.Heat exchangers are typically classified according to flow arrangement and type of construction. The simplest heat exchanger is one for which the hot and cold fluids move in the same or opposite directions in a concentric tube or double-pipe construction.

Figure The two configurations differ according to whether the fluid moving over the tubes is unmixed or mixed.

## Newton's law of cooling

In this case the fluid temperature varies with and. Since the tube flow is unmixed, both fluids are unmixed in the finned exchanger, while one fluid is mixed and the other unmixed in the unfinned exchanger.

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To develop the methodology for heat exchanger analysis and design, we look at the problem of heat transfer from a fluid inside a tube to another fluid outside. It is useful to define an overall heat transfer coefficient per unit length as. We wish to know the temperature distribution along the tube and the amount of heat transferred. For heatingthe heat flow from the pipe wall in a length is. From Next: Generalized Conduction and Previous: Thermodynamics and Propulsion.

The basic component of a heat exchanger can be viewed as a tube with one fluid running through it and another fluid flowing by on the outside. There are thus three heat transfer operations that need to be described: Convective heat transfer from fluid to the inner wall of the tube, Conductive heat transfer through the tube wall, and Convective heat transfer from the outer tube wall to the outside fluid.In mathematics and physicsthe heat equation is a certain partial differential equation.

Solutions of the heat equation are sometimes known as caloric functions. The theory of the heat equation was first developed by Joseph Fourier in for the purpose of modeling how a quantity such as heat diffuses through a given region.

As the prototypical parabolic partial differential equationthe heat equation is among the most widely studied topics in pure mathematicsand its analysis is regarded as fundamental to the broader field of partial differential equations.

The heat equation can also be considered on Riemannian manifoldsleading to many geometric applications. Certain solutions of the heat equation known as heat kernels provide subtle information about the region on which they are defined, as exemplified through their application to the Atiyah—Singer index theorem. The heat equation, along with variants thereof, is also important in many fields of science and applied mathematics. In probability theorythe heat equation is connected with the study of random walks and Brownian motion via the Fokker—Planck equation. In image analysisthe heat equation is sometimes used to resolve pixelation and to identify edges. Following Robert Richtmyer and John von Neumann 's introduction of "artificial viscosity" methods, solutions of heat equations have been useful in the mathematical formulation of hydrodynamical shocks. Solutions of the heat equation have also been given much attention in the numerical analysis literature, beginning in the s with work of Jim Douglas, D.

Peaceman, and Henry Rachford Jr.

### 1. Basic heat transfer

It is typical to refer to t as "time" and x 1The collection of spatial variables is often referred to simply as x. As such, the heat equation is often written more compactly as. In physics and engineering contexts, especially in the context of diffusion through a medium, it is more common to fix a Cartesian coordinate system and then to consider the specific case of a function u xyzt of three spatial variables xyz and time variable t.

One then says that u is a solution of the heat equation if. In addition to other physical phenomena, this equation describes the flow of heat in a homogeneous and isotropic medium, with u xyzt being the temperature at the point xyz and time t.

In mathematical terms, one would say that the Laplacian is "translationally and rotationally invariant. This can be taken as a significant and purely mathematical justification of the use of the Laplacian and of the heat equation in modeling any physical phenomena which are homogeneous and isotropic, of which heat diffusion is a principal example.

This is not a major difference, for the following reason. Let u be a function with.