Heat Transfer using Microchannels.

 What is Microchannel.

A microchannel is a channel with a hydraulic diameter below 1 mm, usually 1–99 μm. Microchannels are used in fluid control and heat transfer  They are more efficient than their 'macro' counterparts, because of a high surface-area-to-volume ratio yet pose a multitude of challenges due to their small size. A heat exchanger is a device that is used to transfer thermal energy between two or more fluids, between a solid surface and a fluid, or between solid particulates and a fluid, at different temperatures and in thermal contact. When properly designed and utilized, micro-channels can distribute the flow precisely among the channels, reduce flow travel length, and establish laminar flow in the channels while achieving high heat transfer coefficients, high surface area-to-volume ratios, and reduced overall pressure drops.

Fig No 1 Microchannel

Types of Microchannels

The shape, size and structures of microchannels vary with different kinds of applications. While most of the microfluidic channels involve a high aspect ratio, low aspect ratio channels are also not uncommon in applications such as particle separation devices.   The method of fabrication also plays an important role. Microchannels are mostly (except micro-milling) fabricated by non-conventional methods. Microchannels with different cross-sections have been fabricated over the years. Most common cross sections include rectangular microchannels, square microchannels, circular microchannels, half circular microchannels, U-shape microchannels and Gaussian beam shape microchannels.

Fig No 2 Rectangular Type Microchannels

Fig No 3 Circular Type Microchannels

Fig No 4 Different Types of Microchannels

Application of Microchannels

Microchannel applications can be divided broadly into three main categories: 

(1) Biological applications 

(2) Chemical applications

(3) Electronics and mechanical engineering–related applications.

 Fig no 5 Microchannels in Air Conditioning

In AC Microchannel coils are constructed of parallel-flow aluminum tubes that are mechanically brazed to heavy aluminum fins. More heat transfer surface is in contact with the air, and high heat transfer rates occur because the refrigerant cools the system and expels waste heat into the air.

Fig No 6 Low Tempratur Radiator

In an indirect charge air cooling system, rather than being released directly to the ambient air, the heat from the charge air cooler first passes through a separate, low-temperature coolant circuit (LT cooling circuit) before being discharged to the ambient air by a downstream, low-temperature radiator (LT radiator). The LT radiator of the indirect charge air cooling system is mounted onto the engine cooling module and can be designed more compactly than the direct charge air cooler, without sacrificing performance. This is because heat is transferred from the air to the coolant. The LT radiator can also be optionally used to ensure optimum thermal management of a temperature-sensitive lithium-ion battery, its power electronics, and soon also the condenser of the refrigerant circuit.

Material for fabrication of Microchannels

Different kinds of materials have been in use for different microchannel-based devices. These materials can be divided into the following three main categories. 

Polymeric and glass substrates 

In the last few years, microfluidic devices have been started to be manufactured on polymeric substrates instead of silicon and glass substrates mainly because of their low cost. The low cost also allows them to be manufactured as a disposable device. The foremost important polymeric materials for microfluidic devices are polymethyl methacrylate  and polydimethyl siloxane .

Metallic substrates

The area in which metallic microchannels have gained considerable attention is its utilization as a cooling device in numerous applications related to mechanical and electronics engineeringIt is also desirable to perform many endothermic and exothermic reactions on such metallic substrates and temperatures can reach as high as 650C. Therefore, the conventional metals such as stainless steel are less preferred for devices. Typical MECS devices include gasoline steam reforming, gas turbine recuperation, flue gas desulphurization and mobile engine heat recovery, requiring temperatures above 650 C in corrosive environments. Many alloys, ceramics and superalloys have been tested, but they are expensive. In this way, metal aluminides have been used as alternative material. Metal aluminides offer high degree of resistance to high-temperature oxidation. This is because of the inherent property of the aluminum (Al) contained in the aluminides to form protective oxide layers (Al2O3). Nickel aluminide (NiAl) is of particular interest because of its high melting temperature (.1600 C). There are also areas where temperature rarely exceeds 150 C such as electronic components. In such situations, it is not very uncommon to use conventional materials such as stainless steel in such devices.

Semiconductors, Ceramics and Composites

 Most of the early developments in microchannel-based applications were based on silicon and were originally developed for integrated circuit (IC) industry. The high-speed digital circuits are required to be cooled at faster rate for performing better with millions of logic gates built over it. For cooling of such devices, either forced air convection technology or liquid-based microchannel technology is used. However, liquid cooling has been proved to be far much efficient than forced air convection cooling. 

Analysis of heat transfer through micro channel by different fluids.

Fluids used for passing through microchannel:

 Ammonia: a) Ammonia is a colorless gas with a characteristically pungent smell. b) It is lighter than air, its density being 0.589 times that of air. 

 Ethanol: a) Ethanol is colorless liquid with the pleasant smell. b) It is completely miscible with water and organic solvents and is very hydroscopic. 

 Methanol: a) Methanol appears as a colorless fairly volatile liquid with a faintly sweet pungent odour like that of ethyl alcohol b) Completely mixes with water. The vapours are slightly heavier than air and may travel some distance to a source of ignition and flash back. 

 Water: a) Warm water vibrates longer than cold water. b) The thermal conductivity of water is high and rise to maximum at about 130°C. c) Water has unusual high viscosity.

PROCEDURE FOLLOWED

The experimentation is carried out for finding average heat transfer coefficients for microchannel through 4 fluids. The step by step procedure adopted is as follows: 

1. Place the arrangement under study in the test section. 

2. Adjusting wattmeter power supplied to microchannel. 

3. Achieving the stable temperature, we will pass fluids from microchannel. 

4. After steady state, calculate the reading of temperature of test section with temperature indicator by thermocouple. 

5. Heat transfer coefficient of conduction and convection is calculated.

RESULTS 

The heat transfer capacity as the function of the operating temperature and the test module is filled with the different working fluids and they observed the heat transfer capacity of these different working fluids. We also observed that except the water other four working fluids shows that, when the operating temperature increases in the heat transfer capacity also increased. And in case of water as the operating temperature increases the heat transfer capacity is rapidly increases. So we can say that the water can gives the best result than the other working fluids. The following graph can shows the operating temperature varies with the heat transfer capacity. 10. 

CONCLUSION 

 Microchannels are capable of removing heat up to 1000 w/ due to their high thermal conductivity. 

 It is clearly seen that for operating temperatures below 50°C, it is more advantageous to use ammonia as the working fluid to maximize the heat transport capacity. 

 When we used water as fluid we can conclude that, as the temperature increased the heat transfer capacity of water is increased. 

 Water is preferable if the operating temperature is higher than 50°C. According to that we can say water can gives good or better result than any other working fluids. 

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