What is Heat transfer: Definition and 935 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.
Considering the approximation of perfect gas, I don't understand why at higher altitude the temperature is lower.
Intuitively it is clear to me, but I do not understand the kind of transformation that takes place; the gas is free to expand and the pressure too is not constant, since it decreases...
Hello all ,
I need to estimate the time it would take for the inside of a hollow cylinder to reach a limit temperature (detailed data below), in order to adjust the wall thickness and choose an insulating material to delay heating as much as possible. Being a total beginner in thermodynamics, I...
A typical UK domestic hot water tank holds c. 200 litres of water. Hitherto the common method of heating it to say 60C has been by means of an internal coil of copper pipe through which is pumped hot water from a gas boiler at ~ 70C. Heat transfer on the outside of the coil is by natural...
Referring to the attached, the bottom right is an electronics compartment. The external panels are 20GA steel powder coated yellow. There is 1-inch of R-6 foam board insulation on the inside of all external walls. The skid is exposed to ambient heat and direct sunlight when on site. If we...
I am constantly research a wide range of areas and often run into questions I cant seem to find the answer to, no matter how much I ask uncle google. I am looking for open dialog and discussion, so I dont mind getting off topic a bit. I like the inspiration. Tho periodically I may need a...
Say I have a real counter flow heat exchanger using air and water. I run a test so I know every parameter; mass flow rate of both fluids, surface area, and in and out temps.
This is great. Now I can calculate the heat transfer rate, my log mean temperature difference, and overall heat...
I have a cube with a volume of 1000m3 at an initial temp of 290K. The bottom side (10m by 10m) is open to the ambient air. I put this cube into a huge fridge and cool the whole volume by 5K. I close the open side by placing a cover on it. This cube has now got a volume of air at a temperature of...
Hi, zero education on heat transfer here, need some help.
I have an electronics module that generates 15 W of electrical power over a 70mm x 20mm x 15mm heat load volume (aluminum). The ambient temperature is 25° C, the maximum temperature the heat sink can be is 40° C (ΔT = 15° C).
Does...
Looking to build a suitable closed loop water (probably glycol mix) circulation system to prevent livestock water troughs from freezing. I assume that I need to calculate how much heat loss is predicted from the water tank in ambient air (varies, but assume worst case), then what length of heat...
Hello everyone,
I'm trying to solve the transient heat transfer problem within the ID wall.
The material is steel, and it is isotropic.
The properties are given below :
L = 5 mm
qin = 0
Tinf = 100 deg C
Tini = 20 deg C
rho = 7850 kg/m3
cp = 460 W/Kg.K
k = 45.8 W/m.K
h = 20 W/m^2.K
alpha = k /...
I'm using a stainless steel pan and a fan to cool my coffee down as quickly as possible.
Usually, I have the fan blowing down towards the coffee, but this morning I flipped it over to suck the heat out of the coffee and I'm wondering which of the two is the most efficient, especially as, in the...
I have some doubts about a thermodynamics exercise I did-I'll write down the text and show how I tried to solve it to see if I reasoned it out right.
In a container of thermal capacity ##C## containing ##1.0 kg## of ice at a temperature of ##-4.0 °C##, a mass of ##3.0 kg## of water is poured...
Hello,
I'm looking over some old school notes and re-learning some basic heat transfer. I have known Fourier's Law (1D: ##q = k\frac{dT}{dx}##) for a long time, but when I look at it now I find it strange that heat flux is inversely proportional to the length of the conductor. I would think...
I have a solar water heater on my deck, with a circulator powered by PV, that pumps heated glycol through a wand heat exchanger placed in an 80 gallon tank. Water comes from the well's pressure tank, passes through this solar pre-heater tank, then into an LPG heater that adds any heat needed. I...
Hi, cannot get a definite answer anywhere on how heat transfer occurs with this fluid. All that the manufacturer gives is "excellent heat transfer". Water has excellent heat transfer due to convection currents. Need to know if this fluid also heats by convection rather than conduction...
***A Carnot Engine*** is a theoretical engine unlike a Sterling Engine which can be made practically.
Some of the drawbacks of Carnot Engine are,
1)The Heat Transfer occurs only during isothermal process(compression and expansion),this is because the working material (ie) gas or fuel used, if...
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.
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...
Hi guys, I'm a 2nd year mechanical student currently taking heat and mass transfer. A question came up in my mind when I was looking into the conduction heat transfer.
If I put block B on block A where block A is 573K and block B is 303K. Is there any way that I can calculate the time required...
I have been given the task of modeling the heat transfer from a light source of known power into a system consisting of two connected materials. I must find the temperature change in the bottom surface. The two materials are initially in thermal equilibrium with the surroundings.
My first...
I believe that just using the equation above should yield the following:
R_{tot} = \frac{18 -(-5)}{400 + 2(80)} = 0.041
but the units don't seem to match up... The units of the above answer are in K / W.
Is there a different formula i should be using?
Your help is appreciated!
Hello everybody, would somebody please put me on the right track to answering this question?
'Consider water undergoes a heat transfer
at constant pressure of 10 MPa and
changes from liquid to steam. Find the
entropy of the system (sfg) as well as the
heat transfer per unit mass in this...
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...
Hi,
starting for this thread Question about entropy change in a reservoir consider the spontaneous irreversible process of heat transfer from a source ##A## at temperature ##T_h## to another source ##B## at temperature ##T_c## (##T_h > T_c##). The thermodynamic 'system' is defined from sources...
In general, in the paper Casimir assumes that between the boundaries of a crystal the heat transfer can be imagined as electromagnetic radiation between two hollow bodies connected by a tube (at least this what I understood from his approach),
I'm reading about Casimir limit (Ballistic heat...
Let's say we have a cylindrical fuel pin with fissile material in the middle, followed by a gas gap and cladding material. It is being cooled by water on the outside. The temperature drop through the fissile material should be parabolic due to heat generation, and the temperature drops through...
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...
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.
Hey guys! This is problem from Callens Thermodynamics textbook and I'm stuck with it.
My goal was to get a expression for the entropy ##S## which is dependent on ##T## so I can move into the ##T-S##-plane to do my calculations:
I startet by expressing the fundamental equation as a function of...
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...
My experiment procedure is that I am using burning charcoal as the heat source, and there are three thermocouples used, first one measuring the temperature of the charcoal, second one measuring the inner temperature of the concrete block, third thermocouple is placed on the top part which is...
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...
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...
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...
Im practicing the questions in the problem book and seem to be getting different answers to the book can somebody check cheers.
[Answers: 57.99 W/m: 1739.7 W: 84.9ºC] textbook answers
A water pipe of bore 65 mm bore and 6mm wall thickness, carrying water at 85ºC is
insulated with one layer of...
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
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...
In answering questions (a.) Why heat intake in this system is
## \Delta Q_{sys} = ( \Delta Q_{hot water} + \Delta Q_{cold water} ) / 2 ##
where.
##\Delta Q_{hot water} = c(T_1 - T_f) ##
##\Delta Q_{cold water} = c(T_2 - T_f) ##
I think T or T_f should be between T_1 and T_2
But why is the...
Hi, I'm looking to find a very very basic model to calculate the heat transfer coefficient in a tube with given heat power exchanged ##Q##. The fluid inside the tube starts sub-cooled (##T_{in}## is known) but exits the tube with a quality of steam around 0.15 (pressure is supposed constant)...
Summary:: Determine the temperature distribution in a bar (very long– 2D) with rectangular cross section, in steady state, with imposed flux at one face, convection at the opposing face (Tinf, h), and imposed temperature (T1) at the two remaining walls.
I am trying to find the analytical...
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?
Hello!
I'm currently studying meridional heat transfer, but I'm struggeling to understand the attached figure.
I don't understand why the ocean contributes more than the atmosphere near the equator and why it suddenly falls off towards the mid-latitudes. Also, why the atmosphere peaks at...
Hi,
I was recently attempting the following problem from heat transfer and I didn't really know how to include the fin amongst the other thermal resistances in the problem.
The key quantities are: ## T_{in} = 20 C ##, ## T_{out} = -5 C ##, ## h_{inner} = 5 W m^{-2} K^{-1} ##, ## h_{outer} =...
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)...
When we measure 'the rate of conduction heat transfer'=Q , we assume that the hot side and the cold side's area are same. But if the both side's area is different to each other, how can i know the rate of conduction heat transfer?
like below figure.
Would you like to help me?? Thanks.