What is Heat flux: Definition and 32 Discussions

Heat flux or thermal flux, sometimes also referred to as heat flux density, heat-flow density or heat flow rate intensity is a flow of energy per unit of area per unit of time. In SI its units are watts per square metre (W/m2). It has both a direction and a magnitude, and so it is a vector quantity. To define the heat flux at a certain point in space, one takes the limiting case where the size of the surface becomes infinitesimally small.
Heat flux is often denoted






ϕ

q


→




{\displaystyle {\overrightarrow {\phi _{\text{q}}}}}
, the subscript q specifying heat flux, as opposed to mass or momentum flux. Fourier's law is an important application of these concepts.

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  1. G

    I Kappa factor in heat equation and heat Flux

    I am doing a project, actually it is a simulation. And we aim to determine the spatial and heat flux evolution of the system. The system consists of two concentric cylinders separated by an insulating material. I change the value of kappa of the insulator but the heat flux remains always the...
  2. fluidistic

    I Energy flux direction in a conducting wire?

    On the Internet, I have read that the energy doesn't flow in the wire, for example in a very simple electric circuit made of a battery and a closed loop. When one computes the Poynting vector ##\vec S \propto \vec E \times \vec B##, one gets that its direction is towards the center of the wire...
  3. P

    Rate of heat flux from from hot gas in a pipe to water outside

    I have solved the first 2 parts. For the 3rd part, I have obtained the equation: T(x) - T0 = (T1 - T0)e^(-Φx/fc), where f = fm in the question. How do I obtain that expression for H? Thank you!
  4. FEAnalyst

    Cooling nozzle - convection power

    Hi, when modeling convective cooling in FEA one needs to define film coefficient ##h## and ambient temperature ##T_{amb}##. The software calculates convective heat flux using this data and current surface temperature ##T_s##: $$q=h(T_{s}-T_{amb})$$ However, modeling of moving cooling nozzle may...
  5. LordGfcd

    What is the continuity condition for the heat flux through a boundary?

    Assume there is a boundary separates two medium with different heat conductivity [κ][/1] and [κ][/2]. In one medium, the temperature distribution is [T][/1](r,θ,φ) and on the other medium is [T][/2](r,θ,φ). What is the relationship between [T][/1] and [T][/2] ? Is it - [κ][/1]grad [T][/1]=-...
  6. SHANKARALINGAM

    Heat Flux Calculation for Titanium & Aluminum

    Hello sir, How to calculate the heat flux of titanium and aluminum?What are the essentials for doing the calculation?We are undertaking a project of multilayer coated cutting tool inserts..So we are in the need of this. Regards, Shankaralingam.
  7. A

    Negative Sensible Heat Flux at Earth's Surface

    I'm a bit confused as to how to imagine a negative sensible heat flux near the Earth's surface. Negative means that sensible heat is moving towards the surface. My book states that that would mainly occur at nighttime. During day there is a positive sensible heat flux and sensible heat moves...
  8. V

    How calculate the Heat flux requiered to evaporate sub cool water

    Homework Statement I was trying to validate a calculation I didusing ANSYS fluent, it's about a flow of water that enters a tube, the tube is 16m long and is being heated with a Heat Flux of q'=1077328,47 [W/m^2], now after the simulation runs and I check the results I find out that the steam...
  9. Z

    Heat flux analysis with transient heat conduction

    Homework Statement I have one dimensional heating system. In the center is heating source which is heating two PVC elements located on both sides of the heating source. Heat distribution in other dimensions is negligible because of insulation. Thickness of one PVC element is 0,051 m and surface...
  10. E

    Calculating Local Heat Flux in a Pipe: Is h = Nu*(k/x) the Correct Formula?

    I am a little unsure how to get started with a homework question. Essentially, I have to calculate the local heat flux at a distance 1.2m (x) along a pipe. I have the fluid's properties and have calculated the Reynolds number, for which I've determined the flow to be turbulent and therefore do...
  11. I

    Constant heat flux or constant temperature

    Hey all, first time posting! So I'm having trouble with understanding the differences between the constant heat flux and constant temperature condition when not in a textbook. Some research I have begun working on involves a strip of heat tape sandwiched between two aluminum plates which are...
  12. MexChemE

    Some questions on heat transfer

    Hello, forum! I'm just starting a new course on heat transfer and we're using Incropera's book. Last time I studied heat transfer was in my transport phenomena course, using BSL, so it was kind of a culture shock using the new book, because the methods used are kind of different in some cases...
  13. P

    Heat flux through a composite wall

    1. State the problem A plane wall consisting of three different materials, each of constant thermal conductivity k. Assume steady-state temperature distribution. a) Comment on relative magnitudes of q(dot)''2 and q(dot)''3 and of q(dot)''3 and q(dot)''4. b) Comment on the relative...
  14. G

    Calculating heat flux (Q) without thermal conductivity (K)

    How to calculate K or Q without one or the other? For example, if the material is unknown (but cannot test density, or melting temperature) how would you test heat flux? Q=K(Th-Tc)/(x)
  15. MexChemE

    Heat and mass transfer -- Boundary conditions & balance terms

    Hello, PF! Recently, while reading chapter 10 (microscopic energy balances) of the second edition of BSL, I found a minor discrepancy which is confusing me, especially when considering the mathematical analogies of heat and mass transfer. In section 10.1, the authors introduce Newton's law of...
  16. A

    Heat flux trough non-homogeneous bar

    When you have the both sides of a non-homogeneous bar with thermal conductivities k1 and k2 in contact to 2 different temperatures, can you consider it the same as if there was some flux going through a homogeneous bar with the weighted mean of k1 and k2 as the conductivity?
  17. U

    Heat equation given constant surface heat flux

    How would I go about finding temperature distribution in a thin square plate during the the first few milliseconds (or actually a fraction of a millisecond) after t=0s. Initial temperature distribution throughout the plate is known, there is heat flux to one side = Qinj, while heat flux from all...
  18. P

    Heat flux through wall question

    Homework Statement I was given this question. The wall of a building is constructed of face brick (10cm thick) and concrete block (50 cm thick) with an air filled cavity (5cm thick). The interior and exterior convection coefficients are 8.3 W.m^-2.K^_1 and 35W.m^-2.K^_1 respectively. The...
  19. A

    Heating a metallic rod from one end by continuous heat flux

    What is the solution of heat transfer equation when there is continuous heating ( flux of energy) to one end of a metal rod? what will be the temperature profile before reaching the steady state? when I try the boundary condition where one end (x=0) is kept at T1, I got the general solution of...
  20. 2

    Model of how temperature depends on latitude?

    Homework Statement I was asked to 'Use a very simple model to find how the Earth’s temperature should vary with latitude.' Homework EquationsThe Attempt at a Solution I was thinking about flux and how this should be proportional to the temperature. So I first thought about the area that is...
  21. G

    Heat Flux Calculation: August-February Net Loss

    If mean seawater temperatures vary from 20C in August to 3C in February, for a patch of water with a depth of 10 meters, what is the net heat flux lost (in W m^-2) to the atmosphere from August to February?
  22. T

    Heat flux with oscillating heat sink

    I am curious about the effects of oscillation in a heat transfer system. For example consider a simple system consisting of a heat source, a heat sink, and some generic material through which we will flow heat. http://tempneff.com/Hflux.png If we stack them all together, without...
  23. C

    Heat flux as a fraction of energy flux due to thermal re-radiation

    Homework Statement Radioactive decay of elements in the Earth's interior results in a mean heat flux through the Earth's surface of 5x10^-2 W/m^2. What is this flux expressed as a fraction of the energy flux due to thermal re-radiation of absorbed solar energy? If radioactive decay were the...
  24. C

    Heat flux on a cylinder with two insulators

    I'm used to problems which ask me to find the heat flux for when, for example I have a very long cylinder covered with an insulator, each with their respective conductivity coefficient. I'd use the formula \frac{\partial Q} {\partial t} =\int -k\vec{\nabla} T \vec {ds}. But now I have a...
  25. I

    Heat Transfer - Heat Flux (thermal Resistance)

    Homework Statement Hi guys, I don't really need help with answering the dollowing question but just have a quick question about it. I solved the first part of the question with an energy balance by letting q_{conduction} = q_{convection} to get the temperature of he outer...
  26. K

    Can TdS be computed using an area, time double integral of heat flux density?

    Can the TdS by a system be calculated by taking the area, time double integral of the heat flux density? If so, is it possible that this double-integral would take on an opposite sign if inside there was a dominating, growing black hole, where, I would presume, heat flows inwards, and not...
  27. C

    How to calculate heat flux in a hot water tank

    Hi. I wish to calculate the heat flux on a bottom plate of a hot water tank. Would simply (m*c*t)/area give me the correct results. Please help?
  28. M

    CHF(critical heat flux),MCHFR(minimum critical heat flux

    CHF(critical heat flux), MCHFR(minimum critical heat flux ratio) how we will find the MCHFR for vs time for coast down transient by drawing channel operating channel and CHF setpoint curves for several time? I really confused about this can you help me about it
  29. C

    How do you calculate heat flux through a piece of metal?

    How do you calculate heat flux through a piece of metal?? Hello, I need to calculate heat flux in W/m2 through a small cylindrical steel bar measuring 12mm length x 2mm diameter. At one end of the bar the temperature is 31C (the heated side) and at the other end it's 28C. How do I calculate...
  30. L

    English expression - not stacionary heat flux

    Hi, I am doing an experiment. I am heating a sample and changing a temperature on one surface of the sample as a sine function of time. Measuring heat flux I am trying to predict heat response of the sample. After some sine cycles heat response is in every next sine cycle the same. How do you...
  31. E

    Heat Flux of BBQ Grill: Estimating Temp. & Length of Handle

    Does anyone have any idea what the heat flux of a barbeque grill might be? I'm estimating the temp. of the meat at 370 K, which is a little larger than that required to grill burgers to well-done. Just need a ballpark figure for the flux...I am trying to make my own grilling utensil and want...
  32. K

    Time for heating to equilibrium with constant heat flux

    I have the following scenario and hope one of you can help me. I need to find an equation describing a heating process. A cylindrical metal of known dimensions and properties is heated from one side while the temperature of the other side is kept constant. I need to find the equation describing...
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