Isı tekniği ile ilgili deneysel dataların bilgisayarla okutulması
Data acquistion and interfacing applications in the field of thermal sciences and fluid mechanics
- Tez No: 14375
- Danışmanlar: PROF.DR. OSMAN F. GENCELİ
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1991
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 43
Özet
ÖZET Isı tekniği ile ilgili deneysel verilerin bilgisayarla okutulmasını amaçlayan bu çalışma dört ana böl timden oluşmaktadır. Birinci bolünde bilgisayarla veri okuma yönteminin önemine değinildikten sonra bu sistemi oluşturan ana elemanlar üzerinde kısaca durulmuştur. Bilgisayarla veri okuma sistemi, ölçme sistemleri ile ilgili olduğundan ikinci bölümde ölçme sistemlerinin genel yapısı, sensörler ve karakteristikleri anlatılmıştır. Ayrıca bilgisayara veri giriş ve çıkışını sağlayan kartların genel yapısı ve çalışma prensiplerinden bahsedilmiştir. Bilgisayarla veri okuma sistemini n en önemli kısımlarından biri yazılım programıdır. Özellikle deneylerde kullanılmak üzere hazırlanmış bir yazılım programı üçüncü bölümde sunulmuştur. Son bölüm yapılan deneylere ayrılmıştır, ilk olarak yarı sonsuz ortamlarda zamana bağlı tek boyutlu ısı iletimi incelenmiştir. Bilgisayarla okunan deneysel değerler ile teorik analizden elde edilen değerler karşılaştırılmıştır. Daha sonra kumun ısı iletim katsayısını, geçici rejimi kullanarak bulan deneyin verileri bilgisayara okutulmuş ve. numunenin ısı iletim katsayısı hesap edilmiştir. Son olarak bir hava filtresinin basınç düşümünün bulunması deneyinde hız ve orifis plakası nın daki diferansiyel basınç düşümü değerleri bilgisayar tarafından okunmuş ve hesaplanan debi değerlerinin karşılaştırılması yapılmıştır.
Özet (Çeviri)
DATA ACQUISTION AND INTERFACING APPLICATIONS IN THE FIELD OF THERMAL SCIENCES AND FLUID MECHANICS SUMMARY Most of“ the measurement systems have presented -the measured value of”a single variable to an observer; i.e. the systems were single input - single output. However there are many applications where it is necessary to know, simultaneously, the measured values of several variables associated with a particular process, machine or situation. Examples are measurements of flowrates, levels, pressures and compositions in a distillation column? temperature measurements at different points in a nuclear reactor core; components of velocity and acceleration for an aircraft. It would be extremely uneconomic to have several completely independent systems and a single multi -input -mult i -out put data acquisition system is used. In this study, measurement systems and elements of data acquisition system are studied, a computer program has been prepared that «takes easier using two data acquisition cards and some experiments have been performed using these cards. The purpose of a measurement system is to present an observer with a numerical value corresponding“to the variable being measured. In general this numerical value ör measured value does not equal the true value of the variable. The measurement system consists of several elements or blocks. It is possible to identify four types of elements, although in a given system one type of element- may be missing or may occur more than once. - Sensing element s This is in contact with the process and gives an output which depends in some way on the variable to be measured. Examples are : thermocouple where millivolt e. m. f. depends on temperature; strain gauge where resistance depends on mechanical strain; orifis plate where pressure drop depends on flow rate. If there is more than one sensing element in a system, the element in contact with the process is termed the primary sensing element, the others secondary sensing elements. - Signal conditioning element : This takes the videflection bridge which converts an impedance change into a voltage change; amplifier which amplifies millivolts to volts. - Signal processing element s This takes the output of the conditioning element and converts it into a form more suitable for presentation. Examples are : analogue to digital converter which converts a voltage into a digital form for input to a computer; a microcomputer which calculates the measured value of the variable from incoming dijital data. - Data presentation element : This presents the measured value in a form which can easily recognised by the observer. Examples are a simple pointer-scale indicater, chart recorder, visual display unit. Measurement system consists of different types of elements. These elements have some static or steady state characteristics! these are the relationships which may occur between the output and input of an element when input is either at a constant value or changing slowly. aD Range : The input range of an element is specified by the minimum and maximum values of input. The output range is specified by the minimum and maximum values of output. b3 Span : Span is the maximum variation in input or output. c'J Ideal straight line s An element is said to be linear if corresponding values of input and output lie on a strainght line. d3 Non-linearity : In many cases the strainght-line relationship defined by theideal straight line is not obeyed and the element is said to be non-linear. Non-linearity is often quantified in terms of the maximum non-linear expressed as a percentage of full-scale deflection. e> Sensitivity s This is the rate of change of output with respect to input. fi Environmental effects : In general, the output depends not only on the signal input but on environmental inputs such as ambient temperature, atmospheric pressure, relative humidity, supply voltage etc. There are two types of environmental inputs. A modifying input causes the linear sensitivity of the element to change. An interfacing input causes the intercept or zero bias of the element to change. g3 Hysteresis : For a given value of input, the output may be different depending on whether input is increasing or decreasing. Hysteresis is the difference between these two values of output. viih5 Resolution : Same elements are characterised by the output, increasing in a series of discrete steps or jumps in response to a continuous increase in input. Resolution is defined as the largest change in input that, can occur without any corresponding change in output. An example is an analogue to dijital converter | here the output dijital signal responds in discrete steps to a continuous increase in input voltage; the resolutin is the change in voltage required to cause the output code to change by the least significant bit. İ3 Wear and ageing : These effects car» cause the characteristics of an element to change slowly but systematically throughout its life. One example is the constants of a thermocouple, measuring the temperature of gas leaving a cracking furnace, changing systematically with time due to chemical changes in the thermocouple metals. The most important element in data acquisition systems is the interfacing cards. The function of the interfacing card is to enable the external system hardware to be associated with the processor section of the computer. A typical arrangement is for the interface components to be treated by the processor as sections of memory, with suitable memory addresses being reserved for these components, so that the transfer of data would be effected by means of sets of signals on the normal address and data buses of computer. In cases where large quantities of data need to be transfered in very short periods of time, the interface data lines may be connected directly to the data lines of reserved sections of memory, with transfer then being effected independently of the operation of the central processing unit. In the data acquisition systems, the signal conditioning elements are necessary to convert sensor outputs to a common signal range, typically 0 to 5 V. The voltage signals are input to a time division multiplexer and the multiplexed signal is passed to a singal sample/hold device and analogue to dijital converter CADO. In cases where all the sensors are of an identical type, e.g. 16 thermocouples, it is more economical to multiplex the sensor output signals. Here the multiplexed sensor signal is input to a single signal conditioning element, e.g. reference junction circuit and instrumentation amplifier, before passing to the sample/hold and ADC. The ADC gives a parallel digital output signal which passes to one of the parallel input interfaces of the microcomputer. Another parallel I/”0 interface provides the address and control signals necessary for the control of multiplexer, sample/hold and ADC. The microcomputer performs whatever- calculations Con viiithe input data} are necessary to establish the measured value of the variable. A common example is the solution of the non-linear equation relating thermocouple e. m. f. and tempereture. The computer converts the measured value from hexadecimal into binary coded decimal form. This binary coded decimal data is written into a computer- parallel output interface. The computer also converts each decade of the binary coded decimal to ASCII form; the resulting ASCII code is then written into a serial and/or parallel output interface. These can transmit written into a serial and/or parallel form to remote data representation devices such as a monitor, printer or host mi cr ocomputer. A computer program called VERITÛP has been prepared so that data acquisition cards can be used efficiently. The VERITÛP program is intended to convert an IBM or compatible computer into a dedicated analogue data collection system, capable of logging and storing analogue data from up to 18 separete sources or channels, 8 of which for thermocouples, at sampling frequencies of 100 per second or slower. The analogue vol t agues from up to 18 channels may be separately logged, i.e. sampled at a pre-defined frequency, converted to dijital form and stored, first in the computer memory and then on magnetic disc. There are actually two seperate routines provided for this, one for fast data collection C sampling rate greater than 1 sample per second) and the other for slower sampling rates. The first routine is optimised for speed, and in the flexibility of data storage options. The slow data collection routine provides a real-time display of the analogue signal values during logging. The computer screen shows the following values during logging. The computer screen shows the following values at all times, for each signal connected : - The current value, in the chosen engineering units. - The maximum and minimum values measured during data logging - The current status of the channel (whether logging lias started, or is currently active, or has finished). - How many samples have been measured and stored. The fast data collection routine provides two of the above options; the current status and number of samples stored. All logged data are stored, at the end of the data collection run, in disc files. Each active channel channel's data are stored as a function of time, and each linked channel as a function of its master channel'sdata. These disc files are suitable for use wiht common proprietary data analysis packagas such as the Lotus i -2-3 spreadsheet and the dBase database programmes. In the last section of this study, some experiments have been performed by using data acquisition cards and VERITÖP programme. In the first experiment, it is aimed to find the distrubition of temperature in a semi -infinite solid with a constant heat flux. For constant properties, the differential equation for the temperature distribution dö â Z9 a ât â x2 where & is the temperature difference between the temperature of solid at x point and initial temprerature of solid. For the s ante uniform initial temperature distribution, it is suddenly exposed the surface to a constant surface heat flux q. The initial and boundary conditions would then became t < 0: t > 0| x = Oj â x X s ûû ; 6 = 0 For- the solution for this case is. 2 q r İ.S2 x x B - - k SCtot/nO expC- D-xerfcC.'3 i ^“2 4,at 2C at 'J where k is the thermal conductivity and os is thermal diffusivity of solid, t is time. This theoretical analysis is compared with the experimental results and it is shown good agreement with these. The second experiment is carried out for assessing the thermal conductivity of sand. The transient needle method is used for this. This medhod is a portable instrument consisting of an electrically heated probe containing a temperature measuring element, and it is constructed to represent as closely as necessary or practicable an infinite line-source of heat.If in a medium of thermal diffusivity CA the temperature is a function only of time t and the distance r from the linear source, the relation between the variables is given by the Fourier equation in cylindrical co-ordinates, d e d t = ot C- d 2ö d r ı d e + 3 r d r Starting from thermal equilibrium with constant rate of haet production during time t the solution k q 4 n EiC - 4 c* t -I can be obtained, in whivh Ei i: exponential integral function. the symbol of the This formula is exact for a probe of infinite lenght and radius r, of material of the same diffusa vi t y and conductivity as the sand in which it is inserted, and having an inf i nitesi mall y thin central heater-wire, provided that the temperature is measured on the surface of the probe and that there is no contact resistance at the boundary between the probe and the external medium. Then, if rz/4Lc&, is sufficiently small, i.e. if, for given values of r and a, t is sufficiently large, 4 n l ce - e 5 1 2 k U I logCt /t 5 ”i 2 This means that if heat is applied at a line source and the temperature 0 at a small radius r is plotted against the logarithm of time which elapses from the moment of load application, the graph should became a straight line, the slope of which directly indicates the thermal cunductivity of the medium. In this experiment, the tempature and power supply voltage are measured by computer and the thermal cunductivity of sand is calculated. Experimental results are shown in figures 4.2, 4.4, 4. 5 and 4. 7.
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