Silindirik geometrideki bir ısıtıcıya dik akış halinde kritik ısı akısının incelenmesi

Abstract

- m - ÖZET Sunulan doktora çalışmasında, silindirik geomet rideki bir ısıtıcıva dik akış halinde, kritik ısı akısı belirlenerek, ısıtıcı çapı» akışkanın hızı, sıcaklığı ve akış yBnü gibi çalışma şartları ile akışkanın fizik sel özelliklerinin etkileri incelenmiştir. Atmosfer basıncında, R-113 kullanılarak yapılar, denevlerdej ısıtıcı çapı 0,5-5 irnm), akışkanın hızı C$i23-ü,7S8 (m/s), sıcaklığı 18,1-46,9 <°C) değerleri arasında değ iş tir ilır.iş ve akış yönü yukarıdan aşağıya doğru seçilini ş t iı. Ayrıca, durgun ortamda da deneyler yapılmıştır. Deney sonuçlarının geı.eli eş t irilmes i ve benzeşim şartlarının belirlenmesi, kritik ısı akısını etkileyen değişkenlerden oluşturulan boyutsuz sayılardan yararla nılarak sağlanmıştır, Etkili boyutsuz sayılar ile ara larındaki bağıntının bulunması için regresyon analizi usulü geliştirilmiştir. Elde edilen bağıntılarda orta lama sapma ±% 10 olup, deney sonuçlarının 2 90*nmda ise en fazla ±2 20 sapma ger ülmüş tür. Altı bölüm ve bir ek halinde sunulan çalışmanın giriş bölümünde, konu tanı 1 ı İniş vt kaynama olayı ile *lgüi geuel bilgiler verîlertk, tarifler yapılmıştır. Kaynak araştırması ve sonuçları ikinci bölümde top la nı& ıştır. Üçüncü böl üs, deney tesisatı, deneylerde izler.ee yol v« ölçülen değerlerin açıklanmasına ayrılmıştır. Doru üncü bölümde hata ;(e«âbı yapılısış, ölçü alet*..tri ve o iç me tekniği dikkate alınarak» ısıtıcı çapının *i' 1., ısıtıcı eksenindeki akışkan tnzınin*% 8« akışkan sıcaklığının t% 0,8 vt kritik ısı skııımn da (-% 8,3, ?*? Z 2,3) »naksimuK bata i 1 1- o İç 01 düğü anlaşılmıştır.* IV - Beşinci bölümde teorik çalışmalar hakkında geniş bilgi ile kritik ısı akısını veren yeni bağıntılar ve- r i İmi ş tir. Sunulan çalışmada elde edilen sonuçlar altıncı bölümde özetlenmiştir» Bu bilgilerin ışığında, - Isıtıcıda üretilen ısının zamana göre değişi minin, kritik ısı akısını etkilediği, - Doyma sıcaklığındaki akışkanın kaynaması ha linde, ısıtıcı çapı büyüdükçe kritik ısı akı sının azaldığı, - Kritik ısı akısının, akışkan hızı ve soğuk kay nama sıcaklığına bağlı olarak arttığı, - Akışkanın yukarıdan aşağıya doğru akması halin de, daha yüksek kritik ısı akısı değerlerine çıkılabildiği b e 1 i r 1 e nm i ş t i r.

- v - SUMMARY III this thesis work, an investigation has been carried out to determine the critical heat flux on cylindrical heaters in a liquid in croas flow at atmos pheric pressure under the conditions as close to the steady state as practical. The independent variables of interest can he divided into two gt&nips. One group con sist of the parameters describing the system, such as» heater diameter, liquid velocity and subcooling, and flow direction. The other group includes the physical properties of the fluid. Because of the lack of complete and detailed in formation on the various variables mentioned above, it was necessary to perform a new series of experiments for pool and forced convection boiling cases. For this purpose, two different types of test apparatus were designed and installed. The pool boiling experiments were conducted in a plexiglas vessel where as for the forced circulation a loop was used to pro vide a liquid stream flowing vertically downward and perpendicular to the major axis of a single, horizon tal, direct current heated tubes and wires mounted in a rectangular flow channel. Refrigerant 113 was preferred as the test liquid for its low latent heat of vaporisation compared with water. Heaters of 0,5 (mm) to 5 (mm) outside diameters were tested over a liquid velocity range of 0,123 (w/s) to 0,788 (m/s), and temperature range of 18,1 (°C) to 46,9 (°C) with the critical heat flux ranging from 0,190x10^ (W/m2) to 0,698xl06 (W/m2). The research work presented in this thesis is arranged in six chapters and an appendix.- VI* The first chapter is concerned with the presen tation of the subject in detail and the explanation of boiling burn-out phenomena so as to clear the meaning of the term `critical heat flux`. Since there was no universally accepted definition of critical heat flux until the past few years, it was essential to be pre cise on the weaning attributed to it in this study. In the second chapter, relavent literature and the experimental results of the previous works are summarized. It was observed that there was no study on downward flow, and also inadequate amount of data were avaiable on the effect of subcooling either pool or cross flow boiling* The detailed description of the test apparatus, experimental procedures used and the results obtained are included in the third chapter of the work. In the fourth chapter, the error analysis of the measurements were made and the uncertainty in the expe rimental results were demonstrated. According to the findings, the maximum uncer tainty associated with the basic parameters are ±1 % for diameter, ±8 % for velocity, ±0,8 % for temperature and -8,3 %, +2,3 % for critical heat flux. The fifth chapter is devoted to establishment of a general representation of the phenomena by equations using the experimental data exist in the literature and obtained from this work. Up to now, some semi- theoretical models have been developed to explain the boiling burn-out pheno mena occuring when the critical heat flux is reached in a vertical upward and cross flow of nearly saturated liquid. However, studies have shown that these appro aches have some limitations. The knowledge regarding boiling burn-out phenomena on the subject studied here is yet insufficient. Because, the various factors involved, and the role they play at burn-out conditions, such as, the number and the distribution of active sites, the frequency of bubble departure, the bubble break-oft diameters and the information on the thickness of the thermal and hydrodynamic boundry layers are not know sufficiently well to built up a valid general theoretical model..-. ^.? -» !' - vii - {.:; The other possible methods of correlation ate the dynamic simulitude derived from dimensional analysis or derived from data plotting, and the regression ana lysis. However, it is found out from the literature search that these three methods have not been applied before for this problem. Consequently, it seemed appro priate to us to correlate the existing data by means of regression analysis technique based on the least mean square fits. Then the present analysis wa« performed by applying this method. In parallel to assumptions used before by other investigators, we were implied to suggest that criti cal heat flux may be written as, q, s f(d,v,6,,r,p,p,,<7,k,c,v,8,,6.,p. ) ^kr sk * s Hb s ps s' d kr rkr where the physical properties of fluid are evaluated at the saturation temperature corresponding to the atmos pheric pressure. The variables combined by using both dimensional analysis and non-dimensionalized technique to yield a set of dimensionless numbers which are listed in the appendix. Because of the fact that dimensionless numbers of importance in cross flow boiling have not been iden tified previously, first of all we had to determine the most significant ones and then find relationship bet ween them by evaluating all experimental data and developing a novel regression analysis technique. A general form of the equation to be determined which defines the relationship between independent variables, X., X«,,.., X and a dependent variable Y may be assumed to be B B B Y ? AX/ X-... X n i â n where A denotes a constant term and B^, 82*...» Bn stand for the exponents, since it has been known to be very useful in many boiling heat transfer problems.- vııı - `*>.;{,* i The above equation was obtained seperatly both upward and downward flow conditions by determining their coefficients for saturated and subcocled boiling through the means of a digital computer. These equa tions represent 90 % of the data with 20 1 securacy and the mean deviation is also with in 10 Z. This is an acceptable precision for boiling experiments. S IX. The results obtained are presented in chapter The following conclusions are reached by this experimental and theoretical research: - It appears that the critical heat flux both for saturated and subcooled boiling cases is related to the boiling heat flux history, and that the shorter the total boiling time the greater the possibility of encountering a pre mature boiling burn-out. - In saturated flow boiling, the critical heat flux varies inversely with the heater diameter, because, as liquid velocity increases the natural convection effect becomes less impor tant, and heater element diameter becomes a significant variable. But for pool boiling, there is relatively little dependence on cri tical heat flux. - In subcooled flow boiling, the effect of heater diameter on critical heat flux is negligiable. But for pool boiling, the critical heat flux varies inversely with the heater diameter. - Critical heat flux increases as the liquid velocity increases in saturated and subcooled boiling, for both upward and downward flow, - The degree of subcooling increases the criti cal heat flux in pool and flow boiling cases. - Both in saturated and subcooled flow boiling, critical heat flux is greater for downward flow than for upward flow. ~ Under the subcooled boiling condition, the£p fft '»A effect of velocity on critical heat flu^t is »ore pronounced in the case of horizontal flow than the vertical flow, - Among the investigated physical properties 'of fluids of interest» viscosity* saturation t?n* perature, critical temperature and pressure &f the fluid were found to be ineffective oa the determination ©f critical heat flux for £he range of examined experimental data. As a concluding remark, ît' may &e indicated that the cited exuations allow a fairly good representation of the existing experimental data* Thej should» ho*?ei?eif» not be regarded as final but b© regarded as a step to* ward betterment and be improved a$ soon as more data for different conditions become avaible*