4 votes

How to determine if mass transfer convection is natural or forced?

The Richardson number $\mathrm{Ri} = \frac{\mathrm{Gr}}{\mathrm{Re}^2} = \frac{\text{buoyancy term}}{\text{flow gradient term}}$ (1) can be used to determine if forced or natural convenction dominate ...
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3 votes
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What is the difference between the wave equation and the advection equation?

As alephzero mentioned the difference is huge. I will explain mathematically and then physically. Mathematically, For any given partial differential equation of the form, $$A \frac {\partial^2\...
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  • 486
3 votes
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convection query

You have the right general approach, and the numbers look reasonable. Your airspeed isn't very fast, so I'm not surprised that the convection coefficient you calculate is less than 10 W m-2 K-1. Note,...
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3 votes
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Heat transfer characteristics in narrow rectangular channel

From "Heat Transfer, 2nd Edition" by A.F. Mills, equation 4.51 gives a formula for the average Nusselt number for flow between two parallel plates. This might be appropriate for your duct because you ...
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  • 2,989
2 votes

Heat transfer for a copper rod

If your temperature sensor is far away from the heat source, the detected temperature may increase even after the heat gun is turned off due to a preexisting temperature gradient evening itself out.
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2 votes

How to consider the dimension of a cylinder when computing its radiative budget?

You certainly need to take account of the shape of the object somehow. As a simple example, consider a sphere of radius $r$ in space where everything except solar radiation can be ignored. The cross ...
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1 vote

Why heat transfer happens only via conduction in this case?

Hot fluid rises. So, if the heating element is at the top of a tank, the hot fluid can't rise more that the top of the tank which means it cannot cause convective flow. This is why a kettle is heated ...
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1 vote
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Mean temperature of a fluid in a pipe

Engineers work in Kelvin. Note that the question does also reference air temperature in degrees C. We were often given temperatures in Kelvin, Fahrenheit, Celcius or Rankine. We got in the habit of ...
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1 vote

Minimizing home heating costs

A suspended ceiling at a height of 2.4m is the most efficient way of saving the heating cost. Because right now most of the heat in your home goes up to the high ceiling. If you don't have access to ...
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1 vote

In this scenario, if I continue to increase the length of heat exchanger how will the temperatures change?

Nothing will change beyond L3. As you increase the length of the heat exchanger, the hot will reach the cool temperature, and beyond that point there will be no change. So beyond L3, the temperature ...
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1 vote

How to visualise/measure airflow in sauna?

You can buy an incense stick, which burns slowly and produces perfumed smoke. you then position the incense stick at different points in the sauna and make a video of the resulting smoke trails.
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1 vote
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How to visualise/measure airflow in sauna?

This NASA article discusses some of the common wind tunnel flow visualization methods. A fog machine generally uses a heated mixture of polypropylene glycol and water. You can test different mixture ...
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1 vote

surface temperature due to constant heat flux

The steady-state heat equation is $\nabla^2 T=0$, where $\nabla^2$ is the Laplacian. In the axisymmetric 2-D case, $T=T(r)$ and $\nabla^2 T=\frac{1}{r}\frac{\partial}{\partial r}\left(r\frac{\partial ...
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1 vote
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Why do we consider the normal component of velocity in a flux and not the original direction of velocity?

The flux of a quantity is by definition the rate per time per area of a given quantity normal to the area in question. If you use the original direction of the particles and this direction is not ...
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How to calculate heat loss across a process pipe with perpendicular air flow

Stand at any center point along the axis of the pipe where the temperature is $T$. Assume that you can obtain $h_i$, the internal convection heat transfer coefficient, $k$ the thermal conductivity of ...
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1 vote

How to calculate heat and mass transfer rates from an enclosed box?

For modelling the rate of air flow through the opening, empirical relationships in the ASHRAE Fundamentals Handbook seem relevant here. Volumetric flowrate Equation 37 on page 25.13 of the 1997 ...
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1 vote
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Heat Transfer coefficient calculation for hot air flow inside rectangular duct

There are several issues going on that I believe are making the model not very accurate. They all in some way relate to your question about the convection: Breaking up the flow into 5 discrete chunks ...
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1 vote
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Convection Heat Transfer

Assuming incompressibility you can work with volume-flow to calculate (a first guess) the velocity, $u$ with the following equations. Probably you have some volume or mass $\dot{m}$ flow entring at (...
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  • 1,202
1 vote

Most effective heat transfer in a vertical pipe with small top/bottom temperature difference?

Some solar water heating systems use a tube with a fluid that evaporates / condenses which moves the heat from end to end as you describe. They are designed to work preferentially in one direction as ...
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1 vote

Characteristic dimension for combining free and forced convection?

Okay, I'm not 100% sure if I got the question right but, to combine natural and forced convection for flow over a cylinder (I assumed horizontal) you have the following formula: $$Nu_{comb}=(Nu_{free}^...
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1 vote

Can I neglect kinetic energy when modeling convective heat transfer for viscous, low mach airflow? Why or why not?

You should be fine to neglect kinetic energy from a thermal perspective. The kinetic energy per unit volume will be $1/2 \rho u^2$, which is about 2800 $J/m^3$ for your conditions. Given that the ...
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