The internal combustion engine in an automobile is a complex piece of machinery made up of 150 moving parts. The cylinder in the IC engine withstands the high explosive force developed during the combustion process and also the very high temperature ranging from 2,0000C to over 2,8000C during operation. Radiators are used for removing the heat developed during the combustion process. The same phenomenon takes place in, railway locomotives, piston-engined aircraft, stationary generating plant or any similar use of such an engine.
Automotive cooling system operates by passing a liquid coolant through the engine block, where the heat from engine is absorbed, then through the radiator where it loses this heat to the atmosphere. This coolant is usually water-based. It is usual for the coolant flow to be pumped, also for a fan to blow air through the radiator. The radiator transfers the heat from the fluid inside to the air outside, thereby cooling the engine.
Automobile radiators are constructed of a pair of header tanks, linked by a core with many narrow passageways, thus a high surface area relative to its volume. This core is usually made of stacked layers of metal sheet, pressed to form channels and soldered or brazed together. For many years radiators were made from brass or copper cores soldered to brass headers. Modern radiators save money and weight by using plastic headers and may use aluminium cores. This construction is less easily repaired than traditional materials.
From the birth of the earliest automobiles to the early 1970s, radiators made from copper and brass were in 100% of cars and trucks. In the 1970s, the radiator environment changed. In the wake of the world oil crisis there were urgent calls for ways to reduce fuel consumption. Major automobile manufacturers in Europe and the U.S. began making cars and trucks with lighter materials.
For radiators, this translated to aluminium, which is one third the density of copper/brass and can handle heat fairly well despite its many shortcomings. In its raw state (although not as radiator strip), aluminium is also less expensive. As a result, over the past 20 years, aluminium has taken first place as the metal for radiators in new cars (56% – 44%), even though copper/brass still holds a two-thirds majority of the overall radiator market. In the aftermarket copper/brass reigns supreme with 89%.
When corroded or damaged, for example, aluminium radiators are far more costly to repair than copper/brass radiators. Moreover, aluminium radiators are particularly prone to coolant-side, pin-hole corrosion. When this occurs, the radiator is irreparable. At recent times the copper/brass industry has identified several new technologies that would make the difference in producing a lighter, stronger, more durable copper/brass radiator.
2.0 BRASS AND ITS PROPERTIES
Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties. Brass is a substitutional alloy. As it is softer than most other metals in general use, brass is often used in situations where it is important that sparks not be struck, as in fittings and tools around explosive gases. Brass has a muted yellow color, which is somewhat similar to gold. It is relatively resistant to tarnishing, and is often used as decoration and for coins. The malleability and acoustic properties of brass have made it the metal of choice for brass musical instruments.
Brass has higher malleability than bronze or zinc. The relatively low melting point of brass (900 to 940°C, depending on composition) and its flow characteristics make it a relatively easy material to cast. By varying the proportions of copper and zinc, the properties of the brass can be changed, allowing hard and soft brasses. The density of brass is approximately 8400 to 8730 kilograms per cubic metre (equivalent to 8.4 to 8.73 grams per cubic centimetre).
Today almost 90% of all brass alloys are recycled. Because brass is not ferromagnetic, it can be separated from ferrous scrap by passing the scrap near a powerful magnet. Brass scrap is collected and transported to the foundry where it is melted and recast into billets. Billets are heated and extruded into the desired form and size.
Aluminium makes brass stronger and more corrosion resistant. Aluminium also causes a highly beneficial hard layer of aluminium oxide (Al2O3) to be formed on the surface that is thin, transparent and self healing. Tin has a similar effect and finds its use especially in sea water applications (naval brasses). Combinations of iron, aluminium, silicon and manganese make brass wear and tear resistant.
2.1 ALUMINIUM AND ITS PROPERTIES
Aluminium is a silvery white metal having the symbol Al and its atomic number is 13. Aluminium is the most abundant metal in the Earth’s crust, and the third most abundant element, after oxygen and silicon. Aluminium is too reactive chemically to occur in nature as a free metal. Instead, it is found combined in over 270 different minerals. The chief source of aluminium is bauxite ore. Aluminium is remarkable for the metal’s low density and for its ability to resist corrosion. Structural components made from aluminium and its alloys are vital to the aerospace industry and are very important in other areas of transportation and building.
Aluminium is a soft, durable, lightweight, ductile and malleable metal with appearance ranging from silvery to dull gray, depending on the surface roughness. Aluminium is non-magnetic and non-sparking. The yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa. Aluminium has about one-third the density and stiffness of steel. It is easily machined, cast, drawn and extruded. Corrosion resistance can be excellent due to a thin surface layer of aluminium oxide that forms when the metal is exposed to air, effectively preventing further oxidation. The strongest aluminium alloys are less corrosion resistant due to galvanic reactions with alloyed copper. This corrosion resistance is also often greatly reduced when many aqueous salts are present, particularly in the presence of dissimilar metals.
Aluminium is a good thermal and electrical conductor, having 62% the conductivity of copper. Aluminium is 100% recyclable without any loss of its natural qualities. Recovery of the metal via recycling has become an important facet of the aluminium industry. Recycling involves melting the scrap, a process that requires only 5% of the energy used to produce aluminium from ore, though a significant part (up to 15% of the input material) is lost as dross (ash-like oxide). The dross can undergo a further process to extract aluminium. Recycling was a low-profile activity until the late 1960s, when the growing use of aluminium beverage cans brought it to the public awareness. In Europe aluminium experiences high rates of recycling, ranging from 42% of beverage cans, 85% of construction materials and 95% of transport vehicles.
Recycled aluminium is known as secondary aluminium, but maintains the same physical properties as primary aluminium. Secondary aluminium is produced in a wide range of formats and is employed in 80% of the alloy injections. Another important use is for extrusion.
2.2 ALLOY OF BRASS AND ALUMINIUM
The typical alloying elements of aluminium are copper, magnesium, manganese, silicon, and zinc. There are two principal classifications, namely casting alloys and wrought alloys, both of which are further subdivided into the categories heat-treatable and non-heat-treatable. About 85% of aluminium is used for wrought products, for example rolled plate, foils and extrusions. Cast aluminium alloys yield cost effective products due to the low melting point, although they generally have lower tensile strengths than wrought alloys. The most important cast aluminium alloy system is Al-Si, where the high levels of silicon (4.0% to 13%) contribute to give good casting characteristics. Aluminium alloys are widely used in engineering structures and components where light weight or corrosion resistance is required.
Alloys composed mostly of the two lightweight metals aluminium and magnesium have been very important in aerospace manufacturing since somewhat before 1940. Aluminium-magnesium alloys are both lighter than other aluminium alloys and much less flammable than alloys that contain a very high percentage of magnesium. Aluminium alloy surfaces will keep their apparent shine in a dry environment due to the formation of a clear, protective layer of aluminium oxide. In a wet environment, galvanic corrosion can occur when an aluminium alloy is placed in electrical contact with other metals with more negative corrosion potentials than aluminium. Aluminium alloy compositions are registered with The Aluminum Association. Many organizations publish more specific standards for the manufacture of aluminium alloy, including the Society of Automotive Engineers standards organization, specifically its aerospace standards subgroups, and ASTM International.
3.0 PROPOSED ALLOYS FOR OPTIMIZED RADIATOR DESIGN
For optimized design of radiator to provide better heat transfer and overall cooling efficiency of radiator, alloy of brass/aluminium was taken and compared with base material (aluminium and cupro/brass).
3.1 Brass/aluminium Alloy – material combination
Copper – 50%
Zinc – 10%
Aluminium – 40%
Table 3.0 Properties of materials used for radiator design
Cupro/brass (70%Cu – 30%Zn)
Density – 8.522E+3
Specific Heat – 385.0
Thermal Conductivity – 111.2
Density – 2.707E+3
Specific Heat – 896.0
Thermal Conductivity – 220.0
Brass/aluminium Alloy (50%Cu, 10% Zn, 40% Al)
Density – 6.274E+3
Specific Heat – 586.8
Thermal Conductivity – 292.22
4.0 MODELLING OF AUTOMOTIVE RADIATOR
Radiator of a vehicle with optimum number of core tubes as per the standards is designed using the modelling software Pro-E.
Pro/Engineer is the software product of PTC (Parametric Technology Corporation). Like other 3D CAD software pro engineer also have different modules for part and assembly. To design a part in pro engineer the part module is selected. For assembly (i.e. group of parts) all the parts are created (or modelled) separately using pro engineer part modules and then in pro engineer assembly module all the parts are called and assembled to create the final assembly.
Modelling in pro engineer or any 3D CAD software is similar to actual manufacturing of the part. Any pro engineer model is build up by adding numbers of features together and these features are actually similar to the operations needed to manufacturing the part.
Dimensions for the model are taken from standards of various leading car and truck producers. Here the Radiator design considered is from the car type BMW. The core dimension as given by the standards of the car type considered is 400 x 718 x 16mm. The header dimension of the radiator is 727.5 x 46.1mm. The inlet and outlet hole diameter is equal and is numerically 39.5mm. The 3D model of the radiator is created using the modelling software Pro-E.
Figure 4.0 2D Diagram of the Radiator (Car Type – BMW)
Table:4.0 Specifications of the radiator chosen
Radiator type – Cross flow, single row core, forced air cooled radiator
In hose barb (inlet) – 6.81mm
Out hose barb (outlet) – 6.81mm
Core rows – 35
Core tube – 1.84mm
Fin density – 1.5mm/fin
Core dimensions (L x H x W) – 510mm x 395mm x 40mm
Tube and fin material – Aluminium
End tube material – Nylon 6,6
5.0 THERMAL ANALYSIS OF AUTOMOTIVE RADIATOR
A thermal analysis calculates the temperature distribution and related thermal quantities in a system or component. Typical thermal quantities are:
- The temperature distributions
- The amount of heat lost or gained
- Thermal gradients
- Thermal fluxes.
Thermal simulations play an important role in the design of many engineering applications, including internal combustion engines, turbines, heat exchangers, piping systems, and electronic components. In many cases, engineers follow a thermal analysis with a stress analysis to calculate thermal stresses (that is, stresses caused by thermal expansions or contractions).
Convection can be specified as a surface load on conducting solid elements or shell elements. Convection film coefficient and the bulk fluid temperature at a surface is speified; ANSYS then calculates the appropriate heat transfer across that surface. If the film coefficient depends upon temperature, a table of temperatures along with the corresponding values of film coefficient at each temperature is specified.
For use in finite element models with conducting bar elements (which do not allow a convection surface load), or in cases where the bulk fluid temperature is not known in advance, ANSYS offers a convection element. In addition, the FLOTRAN CFD elements can also be used to simulate details of the convection process, such as fluid velocities, local values of film coefficient and heat flux, and temperature distributions in both fluid and solid regions.
5.2 ANSYS supports two types of thermal analysis:
1. A steady-state thermal analysis determines the temperature distribution and other thermal quantities under steady-state loading conditions. A steady-state loading condition is a situation where heat storage effects varying over a period of time can be ignored.
2. A transient thermal analysis determines the temperature distribution and other thermal quantities under conditions that vary over a period of time.
5.3 TRANSIENT THERMAL ANALYSIS
The ANSYS Multiphysics, ANSYS Mechanical, ANSYS Professional, and ANSYS FLOTRAN products support transient thermal analysis. Transient thermal analysis determines temperatures and other thermal quantities that vary over time. Engineers commonly use temperatures that a transient thermal analysis calculates as input to structural analyses for thermal stress evaluations. Many heat transfer applications – heat treatment problems, nozzles, engine blocks, piping systems, pressure vessels, etc. – involve transient thermal analyses.
A transient thermal analysis follows basically the same procedures as a steady-state thermal analysis. The main difference is that most applied loads in a transient analysis are functions of time. To specify time-dependent loads, the Function Tool can either be used to define an equation or function describing the curve or then apply the function as a boundary condition, or you can divide the load-versus-time curve into load steps.
- If the Function Tool is used, “Using the Function Tool” in the Basic Analysis Guide is referred for detailed instructions.
- If individual load steps are used, each “corner” on the load-time curve can be one load step.
For each load step, both load values and time values, along with other load step options such as stepped or ramped loads automatic time stepping, etc need to be specified. Then each load step is written to a file and all load steps are solved together.
- Building the model.
- Applying loads
- Obtaining the solution.
- Review of the results.
6.1 BUILDING THE MODEL
To build the model, the job name and a title for the analysis were specified as “Optimisation of radiator Cu-Br”. The ANSYS GUI was used. Then, the following ANSYS preprocessor tasks were followed:
- The element type was defined as Thermal Solid Tet 10 node 87
- Material properties were defined as follows
SPECIFIC HEAT 385.0
THERMAL CONDUCTIVITY 111.2
- The model of the radiator was imported from Pro-E as a IGES file.
- The model was meshed with element size 0.5.
- The file was saved in *.DB and *.Log types separately.
Table:6.0 Element count in tube-fin assembly with 32 fins
Number of hexahedral elements – 469568
Number of prismatic elements – 10304
Number of 2D elements – 168320
Total Number of elements – 648192
Aspect ratio – 2%
Skewness – 1%
6.2 APPLYING LOADS AND OBTAINING A SOLUTION
In the transient analysis, the first step in applying transient loads was to define the analysis type and then establish initial conditions for the analysis.
- The analysis Type was selected as Transient Thermal Analysis.
- The initial temperature of the system was defined as 950C (Temperature of water flowing inside the tube)
- The final temperature of the system was defined as 350C (Temperature of air outside the tube)
- The Thermal Convection On inside Areas were 1200 for ‘Film coefficient’ and 450 for ‘Bulk temperature’
- Then the outside Areas, except the Area which is insulated were selected and given values of 200 for ‘Film coefficient’ and 300 for ‘Bulk temperature’
- The file was again saved in *.DB and *.Log types.
- Then the model was solved for the current LS.
- A Contour Plot of the nodal temperatures at all the substeps (time increment) were generated by reading the appropriate set of results from the Results File.
- The first set of ‘DOF Solution’ and ‘Nodal Temperature’ were obtained from the Contour Plot Nodal Solution.
- The Next Set of solution was also obtained.
- This procedure was repeated until all desired substeps have been viewed.
- The temperatures of particular nodes were also viewed on the plot
- The nodal temperatures were listed and saved to a file for further analysis.
- The transient temperature distribution was also animated over the 20 second time period. Where the ‘Number of animation frames’ of desired value (20 used here), ‘Current Load Stp’, the ‘Animation time delay’ of desired value (0.5 s used here) with ‘Auto contour scaling’ were entered.
- A Temperature vs. Time Plot for certain nodes were generated within the Time History Postprocessor.
- All the values were saved separately for further analysis.
8.0 MATERIAL SUBSTITUTION
The above results for base material 1 (cupro/brass) were recorded and the next analysis for base material 2 (aluminium) was carried out with new set of values as follows:
SPECIFIC HEAT 896.0
THERMAL CONDUCTIVITY 220.0
The analysis was continued with same set of load values (thermal condition) and the results were obtained like that of the previous analysis. The results were recorded separately.
The analysis process was conducted for the new set of material properties for alloy material Brass/aluminium Alloy (50%Cu, 10% Zn, 40% Al) as follows:
SPECIFIC HEAT 586.8
THERMAL CONDUCTIVITY 292.22
The analysis was continued with same set of load values (thermal condition) and the results were obtained like that of the previous analysis. The results were recorded separately for the alloy material.
9.0 COMPARATIVE RESULTS
The thermal analysis of the three materials considered was recorded and their heat flux patterns were compared with each other. From this the overall thermal efficiency of the radiator was analysed for individual materials, which is then compared to identify the material with maximum efficiency.
From the individual results obtained the comparative result was recorded. The comparative results show that the Brass/Aluminium alloy have maximum thermal efficiency preceded by cupro/brass and then by aluminium in the efficiency chart.
The radiator was modeled using 3-D modeling software Pro-E and imported to Ansys as IGES file. Using this Neutral file format the model was meshed in Ansys. The solid model was converted into Finite Element mesh. The material properties were defined to the model. The boundary conditions such as initial temperature of the coolant, final temperature as the atmospheric temperature, thermal conductivity, density, specific heat and the heat transfer coefficient were assigned . The temperatures of particular nodes were viewed on the plot. The nodal temperatures were also listed and saved to a file. The transient temperature distribution was also animated. Heat flux and temperature profiles were used as measure of efficiency. The results were compared and the Brass/aluminium alloy with maximum efficiency is consolidated from the results and reported.
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