1) Thermal anemometre [Group: Measuring apparatus]
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A thermal anemometer measures the velocity at a point in a flowing fluid — a liquid or a gas. Figure 29.17 shows a typical industrial thermal anemometer used to monitor velocity in gas flows. It has two sensors — a velocity sensor and a temperature sensor — that automatically correct for changes in gas temperature.
Both sensors are reference-grade platinum resistance temperature detectors (RTDs). The electric resistance of RTDs increases as temperature increases. For this reason, they are one of the most commonly used sensors for accurate temperature measurements. The electronics circuit passes current through the velocity sensor, thereby heating it to a constant temperature differential (Tv-Ta) above the gas temperature Ta ; and measures the heat qc carried away by the cooler gas as it flows past the sensor. Hence, it is called a "constant-temperature thermal anemometer."
Because the heat is carried away by the gas molecules, the heated sensor directly measures gas mass velocity (mass flow rate per unit area) ρU. The mass velocity is typically expressed as U, in engineering units of normal meters per second, or normal m s-1, referenced to normal conditions of 0°C or 20°C temperature and 1 atm pressure. lithe fluid's temperature and pressure are constant, then the anemom-eter's measurement can be expressed as actual meters per second, or m When the mass velocity is multiplied by the cross-sectional area of a flow channel, the mass flow rate through the channel is obtained. Mass flow rate, rather than volumetric flow rate, is the direct quantity of interest in most practical and industrial applications, such as any chemical reaction, combustion, heating, cooling, drying, mixing, fluid power, human respiration, meteorology, and natural convection.
The thermal anemometer is often called an immersible thermal mass flowmeter because it is immersed in the flow stream, in contrast to the capillary-tube thermal mass flowmeter, another thermal methodology commonly configured as an in-line mass flowmeter for low gas flows. The thermal anemometer has some advantages and disadvantages when compared with the two other common point-velocity instruments — Pitot tubes and laser Doppler anemometers. Compared with Pitot tubes, the thermal anemometer measures lower velocities, has much wider rangeability, and can be made smaller, but it generally has a higher cost and is not recommended for nonresearch liquid flows. When thermal anemometers are compared with laser Doppler anemometers, they have a much lower cost, do not require seeding the flow with particles, can have a faster time response, can be made to have better spatial resolution, and can have a higher signal-to-noise ratio. On the other hand, in nonfluctuating flows, laser Doppler anemometers provide a fundamental measurement of velocity, independent of temperature and fluid properties. For this reason, they are often used to calibrate thermal anemometers.
Thermal anemometers are subdivided into two categories: industrial and research.
(The measurement, instrumentation, and sensors handbook, John G.Webster,1999, pp. 29_18-20)
2)Warp Knitting: [Group: Textile process]
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There are two varieties of knitting weft knitting (usual kind) and warp knitting.A weft knitting fabrics consist of horizontal paralell courses of yarn and requires only a single yarn.By contrast warp knitting requires one yarn for every stitch in the row(course):these yarns make vertical paralell wales.Warp knitting is resistant to runs and is common in lingerie fabrics,e.g.tricot.Warp knitting is generally done by machine whereas weft knitting may be done by machine or hand.Knitting machines use a different mechanical system to produce results nearly identical to those produced by hand-knitting.(Art of Textile Designing,Jennifer Martin)
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The process in which parallel yarns run lengthwise and are locked into a series of loops to get a dimensionally stable fabric is termed as warp knitting. The yarn movement is kept diagonal to interconnect loops of adjacent wales. The warp knits have good crosswise stretch. The needles produce parallel rows of loops simultaneously that are interlocked in zigzag pattern. The stitches on the face of the fabric appear vertical but at a slight angle while the stitches on the back appear horizontally like floats at a slight angle. These floats are known as laps or underlaps which are a distinguishing identification of warp knits. (Refer Fig. 19). For example, tricot, raschel, milanese, simplex, crochet, etc.
(Introduction to Fashion Technology,Pooja Khurana,Monika Sethi, 2007, p. 55)
3)Casting Defects [Group: Failure type]
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Casting often contain various imperfections which often contribute to normal quality variations. These defects not only give a bad appearance to the castings but also decrease their strength and practical utility. Casting defects generally occur due to improper control of the manufacturing cycle.
Defects in castings occur due to different causes. Practically it is quite difficult to establish a relationship between defects and causes. Roughly, casting defects can be classified into the following groups:
-Defects caused by patterns and moulding boxes.
-Defects due to improper moulding or core making materials
-Defects due to improper sand mixing
-Defects due to moulds, cores, runners and risers
-Defects due to improper metal temperature
-Defects due to improper pouring.
(Manufacturing Process, Yazar: H.S. Bawa,P.76)
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Casting of metal involves pouring or injection of molten metal into a cavity of a particular shape where it is allowed to solidify. The cavity or mold may be of an intricate shape so that when the metal is solidified, a part is produced which, with or without further preparation. may be used for its designed purpose. Even those materials considered to be wrought, originate from cast ingots through deformation work in the solid state. The solidification from liquid at pouring temperature to the solid at room temperature occurs in three stages: (i) contraction of liquid metal. (ii) liquid to solid contraction. iii) contraction of the solid to room temperature. The flaws which may be formed during the solidification process are discussed as follows:
(a) Non-metallic inclusions: Non-metallic inclusions' is a general term applied to sand, slag. oxide etc. trapped in the casting. Most of the non-metallics, generally lighter than the molten metal manage to move to the top of the ingot but some are trapped within because they did not have sufficient time to reach the surface before the molten metal above them solidifies. Usually these inclusions are irregular in shape (Fig. 12.1).
(b) Porosity: Porosity is caused by the entrapped gas in the molten metal which gets trapped in the solid casting. The size and amount depends on the gas content of the metal and the rate of solidification of the casting. Porosity may either occur throughout or in some localised areas of the casting. It's shape is spherical or nearly spherical.
(c) Blow holes: Blow holes or gas holes are caused by trapping of air, mold, or core gases and water vapor in the casting during solidification. They occur in single or in clusters and are of smooth, round, elongated, or oval shape of varying sizes. Sometimes an extremely large gas hole appears like a shrinkage cavity, although it differs from a shrinkage cavity in that the ends of the gas hole are rounded and smooth.
(d) Shrinkage flaws: These are cavities formed during liquid-to-solid contraction. Various forms of cavities can occur in a cashrig.
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