My understanding is that whenever there is a temperature difference in a fluid, then there will be convection current. But this is not always the case, in a nonpolar fluid, with the IR camera, I only saw ring patterns spread out around the heat source, but no convection current was seen. Could anyone help explain this? Does the convection only happens under a specific conditions? I really appreciate your help.
Whenever there is a temperature difference within a solid or fluid CONDUCTION takes place. Convection, on the other hand requires movement of the fluid from one place to another. This movement can be caused by the density gradient due to change in temperature and presence of gravity. This happens in case of natural convection. When the fluid is forced by a velocity field, due to pressure difference, the resulting convective heat transfer happening is called forced convection.
The natural convection does not happen in horizontal direction, also it requires appreciable temperature gradient to cause density gradient sufficient enough to make convection currents. It also depends on the height or depth of the fluid for the convection currents to occur.
Once, you dwell into the basic mechanism of forming of convection currents your doubts will automatically be cleared.
temperature difference is not the only factor that determines the onset of convection in the flow. Factors like gravity and other effects (e.g., electric and magnetic forces in specific types of flow) need to be considered too. For example, convection can be suppressed under stably stratified flow even there is a presence of temperature difference in the fluids. Also IR imaging has limited resolution, it may not have the sensitivity to capture some fast and small convection effects...
As answered above by others, the rings which you are observing are indicative of conduction at steady state. If the fluid is stationary, you will not have convection currents.
Are you studying Rayleigh Benard convection ?? Ring pattern reminds me of that! and it does happen under some specific condition
Not enough information has been given for anyone to offer a considered opinion. Apart from having a container where the boundary conditions are such that the basic temperature field is a linear funtion of height with cooler fluid below, then convection currents of some sort will always appear. They may be weak or, if the heating is from below, then currents could be strong if instabilities arise. Localised heating will always yield a flow.
Please remember that the density of water is at its maximum at four degrees Celsius. This makes the conditions of natural convection, even in such an ordinary liquid, not so simple. What to say about solutions or suspensions ...
An additional note: Let's remember that the motion of a liquid is usually observed due to the scattering of light by particles suspended in the liquid. It is known that particles do not scatter light if their size is much smaller than the wavelength of light. For optical observations of the motion of particles suspended in a transparent liquid, larger particles are needed, which do not have to follow the flow of the liquid. This is a real problem in visualizing fluid flows.
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