Monthly Archives: August 2016

Introduction to Heat Pipes

Heat pipes are hollow metal pipes filled with a liquid coolant that moves heat by evaporating and condensing in an endless cycle. A Heat-pipe can be considered a passive heat pump, moving heat as a result of the laws of physics.

As the lower end of the Heat-pipe is exposed to heat, the coolant within it starts to evaporate, absorbing heat. As the coolant turns into vapor, it, and its heat-load, convect within the heat-pipe. The reduced molecular density forces the vaporized coolant upwards, where it is exposed to the cold end of the Heat pipe. The coolant then condenses back into a liquid state, releasing the latent heat. Since the rate of condensation increases with increased delta temperatures between the vapor and Heat pipe surface, the gaseous coolant automatically streams towards the coldest spot within the Heat pipe. As the coolant condenses, and its molecular density increases once more, gravitational forces pull the coolant towards the lower end of the Heat pipe. To aid this coolant cycle, improve its performance, and make it less dependent on the orientation of the Heat pipe towards earth gravitational center, modern Heat-pipes feature inner walls with a fine, capillary structure. The capillary surfaces within the Heat-pipe break the coolants surface tension, distributing it evenly throughout the structure. As soon as coolant evaporates on one end, the coolants surface tension automatically pulls in fresh coolant from the surrounding area. As a result of the self organizing streams of the coolant in both phases, heat is actively convecting through Heat pipes throughout the entire coolant cycle, at a rate unmatched by solid Heat spreaders and Heat sinks.

 

 

Heat pipes are a smart investment if you have a device or platform that needs any of the following support:

  1. Transfer of heat from one location to another. For example, many electronics use this to transfer heat from a chip to a remote heat sink.
  2. Transform heat from a high heat flux at the evaporator to a lower heat flux at the condenser, making it easier to remove overall heat with conventional methods such as liquid or air cooling. Heat fluxes of up to 1,000 W/cm2 can be transformed with custom vapor chambers.
  3. Provide an isothermal surface. Examples include operating multiple laser diodes at the same temperature, and providing very isothermal surfaces for temperature calibration.

There are some universal benefits of how a heat pipe works across almost all applications:

  1. High Effective Thermal Conductivity. Transfer heat over long distances, with minimal temperature drop.
  2. Passive operation. No moving parts, and require no energy input other than heat to operate.
  3. Isothermal operation. Very isothermal surfaces, with temperature variations as low as ± 5 mK.
  4. Long life with no maintenance. No moving parts that could wear out. The vacuum seal prevents liquid losses, and protective coatings can give each device a long-lasting guard against corrosion.
  5. Lower costs. By lowering the operating temperature, these devices can increase the Mean Time Between Failure (MT-BF) for electronic assemblies. In turn, this lowers the maintenance required, and the replacement costs. In H VAC systems, they can reduce the energy required for heating and air conditioning, with payback times of a few years.

Heat Transfer Limitations of Heat Pipes

Capillary Limit:

The ability of a particular capillary structure to provide the circulation for a given working fluid is limited. This limit is commonly called the capillary limitation or hydrodynamic limitation. The capillary limit is the most commonly encountered limitation in the operation of low-temperature heat pipes. It occurs when the pumping rate is not sufficient to provide enough liquid to the evaporator section. This is due to the fact that the sum of the liquid and vapor pressure drops exceed the maximum capillary pressure that the wick can sustain. The maximum capillary pressure for a given wick structure depends on the physical properties of the wick and working fluid. Any attempt to increase the heat transfer above the capillary limit will cause dry-out in the evaporator section, where a sudden increase in wall temperature along the evaporator section takes place.

Sonic Limit:

The evaporator and condenser sections of a heat pipe represent a vapor flow channel with mass addition and extraction due to the evaporation and condensation, respectively. The vapor velocity increases along the evaporator and reaches a maximum at the end of the evaporator section. The limitation of such a flow system is similar to that of a converging-diverging nozzle with a constant mass flow rate, where the evaporator exit corresponds to the throat of the nozzle. Therefore, one expects that the vapor velocity at that point cannot exceed the local speed of sound. This choked flow condition is called the sonic limitation. The sonic limit usually occurs either during heat pipe startup or during steady state operation when the heat transfer coefficient at the condenser is high. The sonic limit is usually associated with liquid-metal heat pipes due to high vapor velocities and low densities. Unlike the capillary limit, when the sonic limit is exceeded, it does not represent a serious failure. The sonic limitation corresponds to a given evaporator end cap temperature. Increasing the evaporator end cap temperature will increase this limit to a new higher sonic limit. The rate of heat transfer will not increase by decreasing the condenser temperature under the choked condition. Therefore, when the sonic limit is reached, further increases in the heat transfer rate can be realized only when the evaporator temperature increases. Operation of heat pipes with a heat rate close to or at the sonic limit results in a significant axial temperature drop along the heat pipe.

Boiling Limit:

If the radial heat flux in the evaporator section becomes too high, the liquid in the evaporator wick boils and the wall temperature becomes excessively high. The vapor bubbles that form in the wick prevent the liquid from wetting the pipe wall, which causes hot spots. If this boiling is severe, it dries out the wick in the evaporator, which is defined as the boiling limit. However, under a low or moderate radial heat flux, low intensity stable boiling is possible without causing dry-out. It should be noted that the boiling limitation is a radial heat flux limitation as compared to an axial heat flux limitation for the other heat pipe limits. However, since they are related through the evaporator surface area, the maximum radial heat flux limitation also specifies the maximum axial heat transport. The boiling limit is often associated with heat pipes of non-metallic working fluids. For liquid-metal heat pipes, the boiling limit is rarely seen.

Entrainment Limit:

A shear force exists at the liquid-vapor interface since the vapor and liquid move in opposite directions. At high relative velocities, droplets of liquid can be torn from the wick surface and entrained into the vapor flowing toward the condenser section. If the entrainment becomes too great, the evaporator will dry out. The heat transfer rate at which this occurs is called the entrainment limit. Entrainment can be detected by the sounds made by droplets striking the condenser end of the heat pipe. The entrainment limit is often associated with low or moderate temperature heat pipes with small diameters, or high temperature heat pipes when the heat input at the evaporator is high.

Vapor Pressure Limit:

At low operating temperatures, viscous forces may be dominant for the vapor moving flow down the heat pipe. For a long liquid-metal heat pipe, the vapor pressure at the condenser end may reduce to zero. The heat transport of the heat pipe may be limited under this condition. The vapor pressure limit (viscous limit) is encountered when a heat pipe operates at temperatures below its normal operating range, such as during startup from the frozen state. In this case, the vapor pressure is very small, with the condenser end cap pressure nearly zero.

Frozen Startup Limit:

During the startup process from the frozen state, the active length of the heat pipe is less than the total length, and the distance the liquid has to travel in the wick is less than that required for steady state operations. Therefore, the capillary limit will usually not occur during the startup process if the heat input is not very high and is not applied too abruptly. However, for heat pipes with an initially frozen working fluid, if the melting temperature of the working fluid and the heat capacities of the heat pipe container and wick are high, and the latent heat of evaporation and cross-sectional area of the wick are small, a frozen startup limit may occur due to the freezing out of vapor from the evaporation zone to the adiabatic or condensation zone.

Condenser Heat Transfer Limit:

In general, heat pipe condensers and the method of cooling the condenser should be designed such that the maximum heat rate capable of being transported by the heat pipe can be removed. However, in exceptional cases with high temperature heat pipes, appropriate condensers cannot be developed to remove the maximum heat capability of the heat pipe. Due to the presence of noncondensible gases, the effective length of the heat pipe is reduced during continuous operation. Therefore, the condenser is not used to its full capacity. In both cases, the heat transfer limitation can be due to the condenser limit.

Vapor Continuum Limit:

For heat pipes with very low operating temperatures, especially when the dimension of the heat pipe is very small such as micro heat pipes, the vapor flow in the heat pipe may be in the free molecular or rarefied condition. The heat transport capability under this condition is limited, and is called the vapor continuum limit.

Flooding Limit:

The flooding limit is the most common concern for long thermosyphons with large liquid fill ratios, large axial heat fluxes, and small radial heat fluxes. This limit occurs due to the instability of the liquid film generated by a high value of inter-facial shear, which is a result of the large vapor velocities induced by high axial heat fluxes. The vapor shear hold-up prevents the condensate from returning to the evaporator and leads to a flooding condition in the condenser section. This causes a partial dry-out of the evaporator, which results in wall temperature excursions or in limiting the operation of the system.

Heat pipe heat sink:

 

 

Graphene under pressure

Small balloons made from one-atom-thick material graphene can withstand enormous pressures, much higher than those at the bottom of the deepest ocean, scientists at the University of Manchester report.

This is due to graphene’s incredible strength – 200 times stronger than steel.
The graphene balloons routinely form when placing graphene on flat substrates and are usually considered a nuisance and therefore ignored. The Manchester researchers, led by Professor Irina Grigorieva, took a closer look at the nano-bubbles and revealed their fascinating properties.
These bubbles could be created intentionally to make tiny pressure machines capable of withstanding enormous pressures. This could be a significant step towards rapidly detecting how molecules react under extreme pressure.
Writing in Nature Communications, the scientists found that the shape and dimensions of the nano-bubbles provide straightforward information about both graphene’s elastic strength and its interaction with the underlying substrate.
The researchers found such balloons can also be created with other two-dimensional crystals such as single layers of molybdenum disulfide (MoS2) or boron nitride.
They were able to directly measure the pressure exerted by graphene on a material trapped inside the balloons, or vice versa.
To do this, the team indented bubbles made by graphene, mono-layer MoS2 and mono-layer boron nitride using a tip of an atomic force microscope and measured the force that was necessary to make a dent of a certain size.
These measurements revealed that graphene enclosing bubbles of a micron size creates pressures as high as 200 mega-pascals, or 2,000 atmospheres. Even higher pressures are expected for smaller bubbles.
Ekaterina Khestanova, a PhD student who carried out the experiments, said: “Such pressures are enough to modify the properties of a material trapped inside the bubbles and, for example, can force crystallization of a liquid well above its normal freezing temperature’.
Sir Andre Geim, a co-author of the paper, added: “Those balloons are ubiquitous. One can now start thinking about creating them intentionally to change enclosed materials or study the properties of atomically thin membranes under high strain and pressure.”

 

Introduction to machine tool / Single point cutting tool

The tool is wedge shape object of hard material. It is usually made from H.S.S. Beside H.S.S. machine tool is also made from High Carbon Steel, Satellite, Ceramics, Diamond, Abrasive, etc. The main requirement of tool material is hardness. It must be hard enough to resist cutting forces applied on work piece. Hot hardness, wear resistance, Toughness, Thermal conductivity, & specific heat, coefficient of friction, are other requirement of tool material. All these properties should be high.

Classification of cutting tools
A] According to number of cutting edge.

Single point cutting tool
It is simplest from of cutting tool & it have only one cutting edge.
Examples – shear tools, lathe tools, planer tools, boring tolls etc.

Multi point cutting tool
In this two or more single point cutting tools arranged together as a unit. The rate of machining is more & surface finish is also better in this case.
Example- milling cutter, drills, brooches, grinding wheels, abrasive sticks etc.

B] According to motion

Linear motion tools – lathe tools, brooches
Rotary motion tools – milling cutters, grinding wheels
Linear & rotary motion tools – drills, taps, etc.

 

Single point cutting tool geometry
The single point cutting tool mainly consist of tool shank & cutting part called point. The point of cutting tool is bounded by cutting face, end flank, side/ main flank, & base. The chip slide along the face.
The side / main cutting edge ‘ab’ is formed by intersecting of face & side / main flank
The end cutting edge ‘ac’ is formed by the intersection of end flank & base.
The point ‘a’ which the intersection of end cutting edge & side cutting edge is called nose
Mainly the chip cuts by side cutting edge.

Terminology of single point cutting tool

Shank – It is main body of tool. The shank used to gripped in tool holder.
Flank – The surface or surface below the adjacent of the cutting edge is called flank of the tool.
Face – It is top surface of the tool along which the chips slides.
Base – It is actually a bearing surface of the tool when it is held in tool holder or clamped directly in a tool post.
Heel – It is the intersection of the flank & base of the tool. It is curved portion at the bottom of the tool.
Nose – It is the point where side cutting edge & base cutting edge intersect.
Cutting edge – It is the edge on face of the tool which removes the material from work-piece. The cutting edges are side cutting edge (major cutting edge) & end cutting edge ( minor cutting edge)
Tool angles-Tool angles have great importance. The tool with proper angle, reduce breaking of tool, cut metal more efficiently, generate less heat.
Noise radius –It provide long life & good surface finish sharp point on nose is highly stressed, & leaves grooves in the path of cut.Longer nose radius produce chatter.