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Can any tube be a barrel? Investigation into innovative heat exchange systems

A thin tube with internal helical fins fastened in a vice.

Internally finned tube. Photo: AGH UST

Can any tube be a barrel? Investigation into innovative heat exchange systems

Although they look like finned gun barrels, they are not good for shooting. Tubes with internal helical fins can efficiently transfer heat to the environment; however, their production consumes a lot of energy and money. An AGH UST scientist has an idea that can reduce the production costs of such systems, and most of all, improve their heat transfer parameters. Can the experience gained during the creation of heat exchangers be helpful in cooling down gun barrels?

Many processes in the pyrotechnics, chemical, and energy industries generate high temperatures. Overheating devices can lead to severe damage, and therefore, to operate fault-free, they need an effective way of heat transfer. The heat flow occurs in the direction from the higher to the lower temperature, which is a feature exploited by cooling systems. Take electric transformers that are submerged in oil vats. The heat generated by the transformer is transferred to the oil and then, through convection and conduction, to the environment. In more advanced systems, the circulation of oil is triggered by a pump that causes it to flow through a heat exchanger. Such devices are usually made up of a bundle of parallelly placed tubes covered with a cilindrical shell. Heat exchange occurs through the walls of the tubes with another fluid or gas that flows inside the shell.

Methods to intensify heat exchange

The greater the temperature difference between the media, the more intense the heat exchange. In addition to the conduction of heat through the wall of the tube, we can distinguish two critical states in the entire system, namely the transfer of heat from the fluid inside the tube to its internal wall and from its external wall to the environment. We can claim that the heat exchange process is as effective as its weakest link’, says Dr Eng. Remigiusz Błoniarz from the Faculty of Metals Engineering and Industrial Computer Science.

The researcher adds that there are several ways to intensify the heat exchange process. For example, we can modify the temperature difference between the medium cooled down inside the tube and its surroundings, releasing heat energy by compressing the former and increasing the flow speed of the gas or fluid outside. Other methods include coating the outer walls of the tube with a layer of a highly conductive material, as well as increasing the surface of heat exchange by means of introducing fins to the outer and inner sides of the tube. The latter also puts the cooling medium into a whirling motion, increasing the surface area of contact between the fluid and the tube wall by pushing the vapour bubbles inside as a result of the centrifugal force. Previous studies have shown that choosing the appropriate helix angle for the fins can manifoldly increase the amount of exchanged heat.

Innovative systems designed at the AGH UST

Dr Błoniarz continues his work that aims to optimise the production methods of internally finned tubes and, most of all, to improve their heat exchange parameters using unorthodox material solutions. As the AGH UST scientist explains, these types of tube are produced on an industrial scale using extrusion, which is energy-consuming and expensive. Another production flaw of this solution lies in the need to use a material with good plastic properties which does not concur with its strength, for example, aluminium.

Dr Eng. Remigiusz Błoniarz, Photo: AGH UST

A young clean-shaven man with short hair is dressed in his work clothes. Technical equipment behind him.

Among other things, the researcher focuses on finding alternative and cheaper ways to produce internally finned tubes:

Already during the work on my doctoral dissertation have I thought to myself: Why can’t we use the drawing method to do this?’, recounts Dr Błoniarz. ‘During this process, the tube is elongated and its diameter is simultaneously reduced. During drawing, when an element with helical channels is placed inside the tube, it will spin. It turned out that if we plan and carry out the process appropriately, it is possible to use this method to create helical fins’.

The AGH UST scientist emphasises that this is only an introduction to a much more groundbreaking finale. Among the advantages of producing internally finned tubes using the drawing process, the researcher also lists the possibility of using complex material compositions, which will render the tubes operational at higher pressures and temperature gradients. To achieve this, Dr Błoniarz intends to test not only traditional materials, such as various types of steel, but also their combinations with other plastics, choosing them on the basis of their strength and good heat conductivity.

The scholar also plans to introduce modifications that will intensify heat exchange between the external wall of the tube and its environment.

I intend to coat the tube with a layer of copper, which is characterised by considerably good heat conductivity, and use it to make the fins. Choosing the right shape of the fins, as well as the appropriate technology to form them, will allow me to direct the flow of the heat stream perpendicularly to the axis of the tube, which will result in much quicker heat transfer’, declares Dr Błoniarz.

Special applications

It is easy to spot similarities between tubes used in heat exchangers and threaded barrels. In both cases, we are dealing with an internal system of helical grooves – in fire arms, their objective is to put the bullet in a rotary motion, which stabilises its trajectory, which then translates to aim and accuracy. Therefore, can the heat exchange system on which the AGH UST scientist is working find application in fire arms? The overheating of the gun barrels as a result of the high combustion temperature of gunpowder gases constitutes a relevant setback as far as their use on the battlefield is concerned.

Dr Błoniarz explains that fire arms producers focus predominantly on making the barrel heat up more slowly.

However, an inverse approach is possible, that is, prompting the same amount of heat to be transferred outside the barrel’, the AGH UST employee suggests. ‘Increasing the outward heat conduction, the entire surface of the barrel will heat up faster, but the inside of the tube will remain colder, as the same amount of heat accumulates more evenly within the material. Simultaneously, the more the external surface of the barrel heats up, the faster the heat will be transferred to the environment. In such a case, it is imperative to ensure that the amount of heat produced does not exceed the amount of heat transferred outward, because as a result, the barrel might weaken and eventually break. Here, the fundamental question pops up: How do I do it? And this is exactly the answer that the research project is trying to find’.

The project focusing on innovative methods to produce a heat exchange intensifying system received funding within the 13th edition of the LIDER programme organised by the National Centre for Research and Development.