# Heat transfer Definition and 149 Discussions

Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation, and transfer of energy by phase changes. Engineers also consider the transfer of mass of differing chemical species, either cold or hot, to achieve heat transfer. While these mechanisms have distinct characteristics, they often occur simultaneously in the same system.
Heat conduction, also called diffusion, is the direct microscopic exchange of kinetic energy of particles through the boundary between two systems. When an object is at a different temperature from another body or its surroundings, heat flows so that the body and the surroundings reach the same temperature, at which point they are in thermal equilibrium. Such spontaneous heat transfer always occurs from a region of high temperature to another region of lower temperature, as described in the second law of thermodynamics.
Heat convection occurs when bulk flow of a fluid (gas or liquid) carries heat along with the flow of matter in the fluid. The flow of fluid may be forced by external processes, or sometimes (in gravitational fields) by buoyancy forces caused when thermal energy expands the fluid (for example in a fire plume), thus influencing its own transfer. The latter process is often called "natural convection". All convective processes also move heat partly by diffusion, as well. Another form of convection is forced convection. In this case the fluid is forced to flow by use of a pump, fan or other mechanical means.
Thermal radiation occurs through a vacuum or any transparent medium (solid or fluid or gas). It is the transfer of energy by means of photons in electromagnetic waves governed by the same laws.

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1. ### Engineering Looking for calculations I could apply to a vacuum-forming machine

So far I have determined the evacuation time, a basic heat transfer between the heating element and plastic sheet, and a hold down force of the forming bed.
2. ### Triangular cavity thermal radiation simulation

Hi, I'm trying to solve a problem involving radiation in a triangular cavity: As you can see, lengths and emissivities of all surfaces are given. For two of them, the heat flux is known and the temperature has to be found while for the remaining surface it's the other way around. I have the...
3. ### Heat sink - calculating temperature at the base

Hi, I've checked the literature but couldn't find any formulas or examples for this particular problem involving heat sink with straight fins: As you can see, I want to assume: - power at the bottom surface - convection (known ambient temperature and heat transfer coefficient) at the...
4. ### Calculating Transient Heat Transfer in Pipes with Thick Walls

Hi Everyone, I am looking to find how much heat can be stored in a concrete pipe of roughly 0.3-0.4m diameter, and an internal diameter of 0.05m. Air will travel through the internal diameter at 500°C and 17.5bar which will provide the heat for the pipes. This system will then be reversed so...
5. ### Heat Exchanger: +40C rise at 16 liters/minute flow rate

Summary:: How to calculate the rate of heat transfer between hot external water and cold internal water in a pipe. The image attached is my attempt at the question however, I find myself stuck trying to work out internal and external heat transfer coefficient.
6. ### Heat Transfer - Critical Radius of Insulation

A steel pipe with foam insulation is embedded in a concrete wall. The steel pipe is carrying cold water and therefore gains heat from outside atmosphere. Heat transfers through the concrete wall, foam insulation and then to the pipe. Is it possible to calculate critical radius of insulation for...
7. ### Engineering Average heat transfer coefficient (forced convection)

So firstly, I don't understand if the mass flow rate is for steam or for water. If it is for water, I know I can find the heat transfer rate using equation:Q=mcdeltaT. But then I don't know how to find h (the average heat transfer coefficient) because I don't know the surface area (As). I can...
8. ### Heat Transfer in thin tubes

Hi, I am trying to understand how I can estimate the time it takes for a fluid at room temperature flowing through a thin capillary glass tube (2 mm ID) connected to an oven to reach the equilibrium temperature (oven temperature). Assuming the oven is preheated and the tube inside the oven is...
9. ### How to calculate the electrical power output using the Organic Rankine Cycle?

Basically this would be a closed loop geothermal system for electrical power generation. The system would consist of 2 Horizontal Wells connected creating a U-shaped closed loop cycle using thermosiphon effect, with constant recirculation. It's not really a conventional geothermal power...
10. ### How do I get the Convection Coefficient for steam in a jacketed vessel?

Please could someone point me in the direction of an equation/ book I can use to obtain the convection coefficient for steam in a heating jacket. Steam is at 1 bar pressure. I think I need to use a dimensionless number. Thanks
11. ### Condenser calculations - uncertainty

A water cooled plate condenser, condensing refrigerant. Heat exchanger (condenser with plates) characteristics: Description symbol value value (converted) number of plates np 28 number of channels nc 27 number of water channels nwc 14 number of flowing channels for refrigerant...

13. ### Relativistic heat conduction

Are there any known instances of heat transfer via conduction or convection happening at relativistic speeds? Is this even possible or is there a non-relativistic limit to how fast heat can transfer in these ways, like how sound can only move so fast?
14. ### Radiative cooling time derivation (for ambient temperature = 0 °K)

as T_∞ = 0 , I use Q=−F\epsilon A\sigma T^4 for this problem as the integration is much easier, so we start with mcdTdt=−F\epsilon A\sigma T^4 rearrange so T is on one side and t is on the other: −mcF\epsilon A\sigma T^4dT=dt on the left side I integrate from the initial temperature (Ti)...

16. ### 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...
17. ### What is the continuity condition for the heat flux through a boundary?

Assume there is a boundary seperates 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]=-...
18. ### Surface temperature-laser radiation

Hello, I'm looking for the governing equations for the laser radiation on a surface. I understand there are several types and methods. I was wondering if there is an equation to show all the parameters in calculating a surface temperature after laser radiation for a specific time. Some of the...
19. ### Where do I put this Server Room's air exhaust?

Hey guys. I have a room which i want to use as a server room. the devices need to work 24/7 and they get pretty hot. so it is imperative to keep the room cool otherwise the devices will be damaged. I have an air vent to bring cold air into the room for the devices so their fans can suck in cold...
20. ### Heat transfer through a MultiLayer Cylinder (find the Temperature inside)

Hello, I am a Mechanical Engineer a little out of practice on Heat transfer. I am trying to solve this problem. It must be solvable but i have yet to find right equations online. I have a multi Layer Cylinder made up of C350 marraging Steel,Zinc Alloy-12 , then 6061 Aluminum T6. All that is...
21. ### Heater Sizing Calculations

I am trying to determine the size of a single cartridge heater or 2 cartridge heaters to heat a large mass of aluminum to 400F. The aluminum block will be sitting inside an insulated (all sides) enclosure. The following are the specs for this problem. - Max surface temp of aluminum block is...
22. ### Tank Heat Transfer

I'm trying to find a heat transfer equation that includes time. Metal tank, partially filled with liquid. I need to find the time it would take for the temperature on the outside of the tank to drop from θ2 to θ1, given thickness, metal and liquid's properties, and tank dimensions
23. ### Temperature change brought about by radiation

I tried modeling the problem quite a few ways. one was to say that the difference between the heat emitted by the room and the heat emitted by the bulbs would equal the heat absorbed by the room, and that could allow us to calculate temperature. This did not work. I'd appreciate your help
24. ### Heat capacity vs Thermal conductivity in a LN2 bath

I worked on a lab experiment that was meant to measure heat capacity but left me with some other questions. The students measured the mass of a cup of liquid nitrogen as it boiled off, recording mass vs time. Then they drop a solid object into the bath, one experiment with a small bit iron...
25. ### Drying of liquid digestate

I am working on a project to dry liquid digestates (residue from the fermentation process of biogas plants). The liquid digestate has a dry matter (DM) content of about 3%. The digestate is primarily made up of animal manure and maize silage. It is a non-newtonian fluid having a viscosity of 4...
26. ### Two or three types of heat transfer?

Heat transfer: Conduction - Convection - Radiation, but I was wondering if conductive transfer is actually radiation transfer, but at contact distances; well not really contact since no material actually touches each other (at a quantum level). So, is conductive heat actually being...
27. ### Heat transfer problem for cooling a solar panel

I am currently trying to solve a problem for my dissertation. Help would be greatly appreciated. :) A silicon solar panel of area 1m^2 is being warmed by the sun. It achieves a constant temperature of 80 degrees celcius. I am trying to cool this panel which is constantly being provided solar...
28. ### Air Compressor Model Analysis

I have an interest in storing, for later use, energy from excess shaft power developed by a wind turbine. Currently I am assuming that the compressor is running in a steady fashion, continually drawing 300K, 1.0 atm. air into the compression chamber and compressing it to 20 atms. before...
29. ### Calculating the Power Dissipation of a Wire

I'm having trouble verifying an experiment I ran to determine the power dissipation of a heating element. 13.15W of power was applied to 3ft nichrome wire. Temperature readings were collected until they stabilized at 128.5F (room temp was 70F). I want to create a mathematical model of the system...
30. ### Double Effect Evaporator

Homework Statement A double effect forward feed evaporator is to concentrate a 12.5% solution of a certain chemical compound "YZ". The feed is to be at and is to be 120 F of such an amount as to contain 6250 "YZ" per hour. Saturated steam is available at 250 F (29.7 psi ). The accepted value of...
31. ### Waterjet Catcher Tank Heating

Here is a fun question for you heat transfer experts. I have a high pressure waterjet firing into a open tank of water. The hydraulic power of the jet entering the tank is known, along with the flow rate of the jet entering into the tank. All of the energy of the jet is dissipated into the...
32. ### Thermodynamics Energy transfer question

Homework Statement In a frictionless piston-cylinder system, there are 3 kg of R-134a initially at 280 kPa and 15 °C. Heat is transferred to the system in the amount 120 kJ. What will the final temperature of the refrigerant be (deg C)? Homework Equations Q - W = ΔU (internal energy) Q - Wb...
33. ### Why is solar radiation equal to long wavelength emission?

In my heat transfer course, I always had to do analysis of long-wavelength radiation between surfaces first, and find the heat radiated from one of the surface. Then, solar radiation is added to the problem, and suddenly it was said that solar radiation=heat radiated from this surface! Does...
34. ### Thermodynamic cycle efficiency

Homework Statement n moles of gas, that follow van der Waals equation are to be employed as the auxiliary system in a circular cycle(parameterized using T_h and T_c as shown on the TS diagram. Calculate the efficiency of the cycle. Homework Equations , , Q = TdS The Attempt at a Solution...
35. ### Heat supply to a system

My book states that Heat and work represent energy in transit. The state of the system undergoes a change after heat is supplied to it and before work is extracted from it because energy gets stored in it. My doubt is here that ,heat has been transferred to the system and it says heat is energy...
36. ### Laminar flow in a tube, heat transfer coefficient-sanity check

Hi there, Hopefully this is a very easy question and you all can just confirm this for me. When calculating heat transfer into a fluid from a heated tube, is it correct to say that the heat transfer coefficient is *not* dependent on the tube diameter? So, if we solve for T_{out}, we get...
37. ### B Conduction vs/or Diffusion

In context of heat transfer, are conduction and diffusion same process?
38. ### Temperature change in a vacuum?

Hello, i'm currently taking some summer classes at my college and was assigned a project to transport vaccines from one location to another (travel time ~2-5 hours.) We have to keep the vaccine within the range of 35-46 degrees Fahrenheit at all times. My group and I came up with the idea of...
39. ### Transport phenomenon

My book states that when a flow around object is considered, Non dimensional momentum flux is defined as the drag coefficient In case of flow through tubes it states The non dimensional momentum flux is defined as the friction factor What do these statements mean? What do they practically...
40. ### Flow inside pipe, heat transfer

Homework Statement A 5m long heated pipe is used heat up water from 15°C to 65°C. Water flow through the pipe is 10liter/min. The heating gives a constant heat flow in all parts of the pipes surface. The inner and outer diameter of the pipe is 30 and 50 mm, respectively. Calculate the heat...
41. ### Heat transfer boundary condition

Homework Statement I am confused on how it's using the surrounding temperature minus the surface temperature as its the other way around in the Newton's law of cooling, Doing that would change the sign of convection right? I dont see the reason to do that, since if left side is hotter, then...
42. ### Steady state heat flow: radiation and conduction

Homework Statement One end of a solid cylindrical copper rod 0.200 m long and 0.0250 m in radius is inserted into a large block of solid hydrogen at its melting temperature, 13.84 K. The other end is blackened and exposed to thermal radiation from surrounding walls at 500.0 K. (Some telescopes...
43. ### Derivation process? (Heatsink Fin Heat Conduction Equations)

Homework Statement I dont understand the derivation of the right side of the last equation. Homework Equations The Attempt at a Solution I got to this point, I also dont understand why it did not include C_2 for the variation of temp. along the fin. I am guessing the right side is the...
44. ### What happened to the negative sign?

Homework Statement Homework Equations The Attempt at a Solution I thought it would become d/dx(-kAc*dT/dx)+hp(T-T)=0 instead of where the equation in the red box? I dont see how are they equivalent [/B]
45. ### Heat Transfer through multiple walls

Hi guys, I am newbie on thread and currently working on a project I need some help on the Heat flow on multiple walls. Here is the setup (see figure below) The first wall is a Steel material which has a 350C temperature(1kwatts) and the an Air Gap is present then followed by a Plastic wall...
46. ### How long to heat a given volume of air?

Homework Statement I am working on a problem where I need to determine the time taken for an air heater with an 8kW output to heat a volume of air of 9m^3 by 25C. I am neglecting all losses at the moment. It almost seems too simple in my mind that I'm concerned I'm overlooking something...
47. ### Fluid mechanics/heat transfer - meaning of Nusselt number

The Nusselt number represents the ratio of heat transfer by convection relative to conduction. At the onset of convection does this correspond to a Nusselt number of 1?
48. C

### Watts transferred to water?

My colleague has been developing a hybrid water/forced air heat transfer system for a prototype lighting solution we are developing. I am struggling to get my head around the calculations required to figure out exactly how many watts of energy (heat in this case) are being transferred to the...
49. P

### Thermal radiation inside a cooler

I am trying to calculate how much a foil liner helps keep the inside of a package cool. I have calculated the rate of conduction but am now concerned with radiation. Above is the equation I am using. The emissivity for the material is 0.05, the area is 1 m^2, the outside air temp is 295...
50. ### Heat transfer into a sphere

Hi, I am looking to simulate a very - seemingly - simple case. Any advice on a software package would be helpful - preferably gui which doesnt have a steep learning curve. I want to model the heat flux into a sphere from the outside. The dimensions of the sphere are not important to me. I...