Microchannels Heat Exchanger

Research on heat transfer characteristic inside microchannels has become important topic as the advancement in the field of micro-scale technology such as micro electro-mechanical system. The impacts of miniaturization of the system involving high heat transfer processes create challenges for better system’s thermal management which can only be solved by enhancement of the performance and efficiency of applied heat exchanger devices. Microchannels have been considered as the potential solution for such devices due to their high thermal performance characteristic with low space requirements.

As for current development, the fabrication methods of microchannels, which limit the microchannels applications in the early developments, have emerged beyond the ability to model the thermal system on the microchannels of such scale. Micromachining of silicon wafers, micro-extrusion of aluminum elements [1], anisotropic wet chemical etching and sawing, anisotropic dry etching, hybridization and system-on-chip integration [2] are examples of micro-fabrication methods capable of manufacturing microchannels.

Early research in microchannels heat exchanger can be traced back to works by Tuckerman and Pease [3] for a very-large-scale-integrated circuits cooling. In fact, microchannels application can be found in cooling devices for computer’s microprocessors and electronic components, laser diodes, radar and aviation components, and microchemical reactors. For larger system, microchannels have been employed in compact heat exchanger for air conditioning system in automotive and domestic applications. Various microchannel configurations are used in these applications, including important feature of refrigerant headers which provide minimum effects on the upstream flow during two-phase flows inside the microchannel passages.

Despite the extensive applications of microchannels in heat exchanger devices, as for current applications, the devices are being implemented based on extensive testing which are expensive and time consuming. There has not been a fully-reliable thermal design methods for such applications due to the limited understanding in thermal-hydraulic phenomena occurred in micro-scale channels, importantly for two-phase flow boiling heat transfer mechanism.

Important issues on microchannels include two-phase flow patterns, flow boiling heat transfer mechanism and model developments, two-phase pressure drop and prediction methods, and critical heat flux.

References:
[1] Thome, John R., 2004. Boiling in microchannels: a review of experiment and theory, Int. J. Heat and Fluid Flow 25 pp. 128-139
[2] Kandlikar, Satish G. and Grande, William J., 2003. Evolution of Microchannel Flow Passages-Thermohydraulic Performance and Fabrication Technology, Heat Transfer Eng. 24(1) pp. 3 – 17
[3] Tuckerman, D.B. and Pease, R.F., 1981. High Performance Heat Sinking for VLSI, IEEE E. Device Letters, EDL-2 pp. 126-129

Refrigeration and AC contribution to Climate Change

Refrigeration and air conditioning (RAC) system contribute to climate change through their greenhouse gases emission by two means.

First, they emit refrigerant gases from leak and losses in charging/discharging processes. Some of these refrigerants are not only ozone depleting substances regulated in Montreal Protocol, but also greenhouse gases listed in Kyoto Protocol. For example chlorofluorocarbon, CFC. CFC-11 and CFC-12, which are phased out under Montreal, have Global Warming Potential, GWP 1400 and 4100, respectively. These GWPs are based on CO2 which GWP is defined as 1.

However this contribution is relatively low. The bigger contribution for climate change, accounts for 80% of their contributions, come from carbon dioxide emissions which are released from their energy uses. These emissions are associated with the amount of greenhouse gases emission from the sources of power to run the RAC systems, generally from electric power generators for industrial and domestic uses, and some from combustion engines for automotive air conditioning and local uses.

So, the RAC industry roles in curbing the climate change issues should be focused on reducing the energy consumption of their equipments and systems. It can be achieved through increasing their systems’ energy efficiency and developing new technology.
The research and development of alternative refrigerants as the replacement of phased out refrigerants are still needed to be done, due to their importance on keeping the ozone layer safe. However there should be considerations that the replacement should also be associated with maintaining or increasing their systems’ energy efficiency. Reducing RAC systems’ efficiency is simple not an option.

Finally, as an RAC engineers, these are the challenges we need to face today, and these are the ways you can do to contribute for the solution of climate change.

Climate Change: Voices from Yeosu

‘I’m worry,’ said Kyeong-ran, a middle school teacher in Yeosu when asked about her first impression on ‘climate change’. Her opinion was also shared with Yun-jeong, a Yeosu City public servant and Hyun-mi, a university student studying English.

The topic of climate change and global warming warmed the atmosphere of the discussion that night, even it was a pretty cold night in the middle of autumn in Yeosu.It is interesting to hear what people of Yeosu say about climate change. A relatively small city lying about midway along the southern coast of South Korea, beautiful sea and coasts with more than 300 islands and rich of seafood and fish resources, Yeosu is vulnerable with the impacts of climate change. Beside that, Yeosu is also the place of Yeochon industrial complex, including oil and petrochemical plants, claimed as big contributors of greenhouse gases.

However, not everyone in the room was familiar with climate change issues. At least that what I thought when Yang-hyon, a consultant in machinery supplier, and Mi, a final year student majoring in statistic, said ‘I have no idea,’ prior to the discussion. At the beginning, we read a short article about climate change which I had prepared before. Discussion continued with the climate change impacts to their life. Hyun-mi said the temperature was higher these days, even during the winter, compared to the past. The snow was hardly seen again, the spring and autumn periods were much shorter now. Yang-hyon told the story about an industry plant which had to be closed due to environmental concerns. Yun-jeong, who works in agriculture government office, and Hye-sung said about heavy rain and storm which were more frequent nowadays. Even though they were not sure whether the cause was climate change or something else, but they all agreed that human activities had done negative impacts on the environment and we should do something about it.

Those brought our discussion into what efforts are needed to address the negative impacts above. Yang-hyon and Yun Mi argued that the government role is significant to educate the people about climate change issues. Yun Mi stressed the importance of such education on children and young generation to raise their awareness. Yun-jeong suggested active participation of the community members to get informed and to disseminate the information through public advertisements or campaigns. Yang-hyon added the need to apply stricter regulations and penalties for industries, while Hye-sung said about research and development to improve industry process and products’ energy efficiency such as electric or hybrid car in automotive industry. In the short article, I also included individual efforts that can contribute to the solution of climate change problems.

The discussion then continued on such individual efforts and their willingness to do them. They said the importance of each of us to get informed and learn about climate change issues, for example through this discussion and other sources.Energy efficiency in daily life was also brought to our attention. Through lowering electric bill and the use of energy-efficient utilities in the household e.g. fluorescent lamp, freezer, and air conditioning equipments. We also discussed the efforts on limiting the greenhouse gases emission such as using fuel-efficient vehicle and joining carpool or mass transportation. Hye-sung said he planned to change his car into gas-fuelled with higher fuel efficiency and lower emission. Or even into electric or hybrid car but, he complaint, they were too expensive for him. Other efforts which also came to light during the discussion were the 3R steps, reduce, reuse and recycle waste. They agreed these could be applied in daily life and they were willing to do them.

The discussion had to end because it was already late. I, personally, hope that the discussion could benefit them through the information and opinion we shared. One thing which was encouraging from the discussion, was that everyone wanted to contribute to the solution of the climate change problems. Their arguments were not only limited to the government and industry leader roles, but also their own roles and efforts to get the solutions. They are willing to do the efforts, even if they have to change their individual habit or lifestyles. As Yang-hyon said, ‘it’s inconvenient for us to change, but we have to do it’.

Finally, as the recent international poll said that most people are ready to change their lifestyle to curb the climate change problems, I can proudly say, the voices I heard from Yeosu are parts of them.

What you can do:Climate change is a global issue. It is everyone’s problem, wherever you live, no matter who you are. It affects all of us. So it is our responsibility to curb the climate change problems. It is not as difficult as you think; in fact, you can do it in your daily life. Here are some suggested actions:

1. Reduce, Reuse, and Recycle what you use.The steps reduce greenhouse gases released during waste processes.
2. Be energy efficientLower your electricity use, turn of unused heater or air conditioner, utilize energy-efficient utilities e.g. fluorescent lamp.
3. If you want to buy a car, choose one that is highly fuel efficient. And when you have one, don’t use it too often, join a carpool, and take mass transit, walk, or ride a bike — anything that reduces the amount of gasoline you burn.
4. Eat wisely. Choose foods that are local, organic and low on the food chain whenever possible. Make the most of seasonal foods.
5. Educate yourself. Get informed about climate change, sustainable living, participate actively in forum etc.

If those activities seem too hard for you to do, let’s think this way. Those efforts are not only for us, but they are for our next generations, our own children and grandchildren. They will thank us in the future.

For GwangjuNews, Dec 07