# Heat transfer and Fluid Mechanics, COMSOL

• Windforce
In summary, the conversation discusses the attempt to couple heat transfer and fluid mechanics in a model of a concrete slab with a water pipe underneath. The heat flux is applied to the surface of the concrete slab, while the other faces are thermally isolated. The temperature of both the concrete and water at the inlet is set to 273K, and the outlet is set to a convective flux. The conversation also mentions the use of Direct Spooles for a stationary solution and the attempt to find a solver for a time-dependent solution. The speaker also seeks advice on how to improve the analysis, including increasing the mesh size and using artificial diffusion. They also mention potential issues with the velocity and temperature fluctuating along the water pipe. Finally, the conversation

#### Windforce

Heat transfer and Fluid Mechanics, COMSOL Help

Hello,
I am trying to couple heat transfer and fluid mechanic together, model is a simple concrete slab (3'x6')with a water pipe beneath it and along 3' direction. For heat transfer, the heat flux will apply on the surface of concrete slab, other faces are thermally isolated, the temperature of concrete and water at inlet are 273K, and water outlet is set to be convective flux. activate convective heat transfer in water pipe with u, v, w.
For fluid mechanics, DE-activate concrete slab properties, and only activate water pipe, and inlet of water is set to be laminar flow with V=0.058m/s, and outlet is set to be Pa=0.
I tried both stationary and time-dependent,
I used Direct Spooles for stationary, the velocity field at center of water pipe is fluctuate, and as well as temperature, both number will move up and down along the water pipe.
For time-dependent, I could not find a solver to solve it, always show errors on "zero in diagonal matrix".
What should I do to improve the analysis? I increased mesh side, swept mesh, to very fine mesh size, still not work, I also try to use artificial diffusion, Isotropic, but the Vmax will not be V=0.058*2.
Can anyone give me some advice or point out what I did wrong? please, this has been puzzled for couple months,
Thanks

Last edited:

Windforce said:
Hello,
I am trying to couple heat transfer and fluid mechanic together, model is a simple concrete slab (3'x6')with a water pipe beneath it and along 3' direction. For heat transfer, the heat flux will apply on the surface of concrete slab, other faces are thermally isolated, the temperature of concrete and water at inlet are 273K, and water outlet is set to be convective flux. activate convective heat transfer in water pipe with u, v, w.
For fluid mechanics, DE-activate concrete slab properties, and only activate water pipe, and inlet of water is set to be laminar flow with V=0.058m/s, and outlet is set to be Pa=0.
I tried both stationary and time-dependent,
I used Direct Spooles for stationary, the velocity field at center of water pipe is fluctuate, and as well as temperature, both number will move up and down along the water pipe.
For time-dependent, I could not find a solver to solve it, always show errors on "zero in diagonal matrix".
What should I do to improve the analysis? I increased mesh side, swept mesh, to very fine mesh size, still not work, I also try to use artificial diffusion, Isotropic, but the Vmax will not be V=0.058*2.
Can anyone give me some advice or point out what I did wrong? please, this has been puzzled for couple months,
Thanks

try to find a singularity in the fluid or thermal solution and start with a good first guess at the problem solution by running no flow and feed the temperature solution for a first guess

turn smoothing of corners on and refine your mesh and raise your iteration at the beginning of the solution... take like five time steps and plot the solution you may find where the matrix is singular or trouble is occurring

## 1. What is heat transfer and fluid mechanics?

Heat transfer and fluid mechanics are two closely related fields of study in physics and engineering. Heat transfer involves the movement of thermal energy from one location to another, while fluid mechanics deals with the behavior and properties of fluids (liquids and gases) in motion.

## 2. What are some real-life applications of heat transfer and fluid mechanics?

Heat transfer and fluid mechanics are used in a wide range of industries and technologies, including heating and cooling systems, power plants, chemical processing, aerospace engineering, and environmental engineering. They are also important in understanding natural phenomena such as weather patterns and ocean currents.

## 3. How can COMSOL be used in studying heat transfer and fluid mechanics?

COMSOL is a software platform that utilizes numerical simulation and modeling to solve complex physical problems. It can be used to study heat transfer and fluid mechanics by creating virtual models and simulating different scenarios to analyze the behavior of fluids and heat transfer in various systems and environments.

## 4. What are some common challenges in studying heat transfer and fluid mechanics?

One of the main challenges in studying heat transfer and fluid mechanics is the complex nature of the equations and phenomena involved. It requires a deep understanding of mathematical concepts and the ability to apply them to real-world situations. Additionally, factors such as turbulence, non-Newtonian fluids, and non-uniform temperature distributions can make the analysis more challenging.

## 5. How can knowledge of heat transfer and fluid mechanics be applied in solving real-world problems?

Understanding heat transfer and fluid mechanics is crucial in designing and optimizing various systems and processes, such as heat exchangers, engines, and pumps. It also plays a significant role in developing sustainable and efficient energy solutions and in addressing environmental concerns related to fluid and heat transfer in natural systems.