Eksperimentalna i teorijska analiza modela zagrijavanja vode zračenjem

Ferdelji, Nenad (2012) Eksperimentalna i teorijska analiza modela zagrijavanja vode zračenjem. = Experimental and theoretical analysis of radiative water heating model. Doctoral thesis , Sveučilište u Zagrebu, Fakultet strojarstva i brodogradnje, UNSPECIFIED. Mentor: Galović, Antun.

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Abstract (Croatian)

Zagrijavanje vode nezaobilazan je dio tehničkih sustava većine industrijskih i kućanskih objekata čija implementacija sa sobom donosi niz praktičnih problema. U sustavima za zagrijavanje vode, javlja se, naime, problem izdvajanja kamenca na stijenku uronjenog grijača, što ima za posljedicu veću potrošnju električne energije i kraći vijek njegova trajanja, a kontinuirano korištenje dodatnih tvari za sprječavanje taloženja kamenca dodatno poskupljuje rad uređaja. U radu se predlaže idejno rješenje grijača vode, koje bi utjecaj spomenutih nedostataka svelo na što manju moguću mjeru. Originalna ideja takvog rješenja svodi se na izvlačenje grijača iz volumena vode, tj. eliminira se izravni dodir vode i grijača, pa na taj način dominantni model konvektivnog prijenosa topline kod uronjenog grijača, biva supstituiran zračenjem. Prijenos topline zračenjem najkompleksniji je način prijenosa, jer svoju osnovu ima u svojstvima elektromagnetskih valova, čija dualna priroda, i dandanas intrigira znanstvenike širom svijeta. Ovim se radom ne ulazi u samu prirodu elektromagnetskog zračenja, već se pokušavaju odrediti bitna obilježja sustava zagrijavanja u kojem je prijenos energije dominantno zračenjem, kako bi se na provjerenim zaključcima mogao nastaviti razvoj koncepta novog grijača vode. Time se nameće glavni zadatak rada, a to je definiranje i istraživanje utjecajnih faktora na čijim temeljima počiva efikasnost tako koncipiranog načina zagrijavanja vode. U tom smislu, nametnuta su dva bitna faktora koji najviše utječu na iskoristivost zagrijavanja vode, debljina sloja vode i temperatura grijača. Zbog ograničenog izvora informacija na tu temu, eksperimentalna analiza zamišljena je kao faktorski plan pokusa koji, osim utjecaja glavnih faktora, otkriva i utjecaj njihovih međusobnih interakcija. Statističkom analizom dobivenih rezultata otkrivaju se područja viših i nižih iskoristivosti zagrijavanja, kao posljedica varijacije spomenutih faktora. Kako bi se konačno dobila informacija o glavnim pojavama unutar takvog sustava, napravljena je numerička simulacija jednog stanja pokusa. Dobiveni rezultati analize pokazali su ograničenja samog softvera za simulaciju, ali i otkrila moguća poboljšanja sustava zagrijavanja vode. Izvedeni bi zaključci iz provedene eksperimentalne i numeričke analize, mogli poslužiti kao čvrsti temelj budućim istraživanjima.

Abstract

Water heating is an unavoidable part of technical systems in most industrial and household facilities related to a large number of problems in practice. In water heating systems, the problem is the scale deposition on the wall of immersed heater, with a consequence of increased electrical energy consumption and a shorter life of the heater. In addition, the use of agents to prevent scale deposition contributes to an increase in operating costs of the device. A conceptual design of a water heater, which would reduce the influence of the above mentioned weaknesses to a possible minimum, is proposed in this thesis. The original idea of such design is to take the heater out of the water, i.e. a direct contact between water and the heater is eliminated. Thus, the dominant model of convective heat transfer in the case of immersed heater is substituted by radiation. Factors that would decisively influence the heating of water by radiation are assumed based on the theory of the complex phenomenon of radiation, which is described in detail in the second chapter of the thesis. These factors are: • water layer thickness and • temperature of the heater. Consequently, the main aim of the thesis is to define and investigate the influence of these two factors on the heating of water by radiation and to obtain the values of absorption factors, experimentally and numerically. Absorption factors provide the information about both the absorbed energy and about the integral effect of water heating. The concept of experimental analysis together with results of measurements is given in the third chapter. For that purpose, an experimental line to investigate the dynamics of water heating is designed and constructed and described in detail in the fourth chapter. Data-acquisition equipment and software, to complete the tasks of both collecting relevant data and of ensuring good control of electrical energy source, was selected. Experiments were carried out with pure water whose conductivity did not exceed 3 μS. Standard heaters, whose spectral distribution of emitted radiation can be assumed as being in accordance with Planck’s law, were used as a source of radiation. Since a search of recent literature has not resulted in the acquisition of data about the dynamics of water heating by radiation, an experimental analysis was carried out in two parts. In the first part, screening experiments were set and carried out to get a general idea about the influence of the selected factors, and in the second part, i.e. during the main experiment, their influence was determined quantitatively. Due to very complex optical phenomena occurring in radiative heat transfer, both experiment parts were conceived as factorial design experiments to detect possible interactions between the two factors, which has been confirmed by this thesis. In the screening experiments, a big influence of selected factors was determined, with the coefficient of determination of more than 99 %, which accounted for the second part of experimental analysis, i.e. setting and carrying out the main experiments. Another factor was added and investigated during the main experiments. That was • the emission factor of the cover surface. The main experiments were designed as factorial experiments with three factors that were investigated: the water layer thickness investigated as a numerical factor at four levels (5 mm, 15 mm, 30 mm, and 80 mm), the temperature of the heater also investigated as a numerical factor at four levels (1300 oC, 1450 oC, 1550 oC, and 1750 oC), and the emission factor of the cover surface investigated as a categorical factor at two levels (glossy and black). In order to ensure a good quality statistical analysis, each condition in the experiment was repeated four times, which means that 128 measurements were carried out during the main experiments. The output quantity for each experimental condition was the heating efficiency defined as a ratio between a measurable amount of heat transferred to water and a measurable amount of the heat produced by the heater that was used for that purpose. To calculate the heating efficiency, the power of the heater was measured, together with the increase in the internal energy of water and losses by convection and radiation from the tested housing. Finally, the measurement uncertainty was determined for each measurement. Results obtained in the main experiment and presented in the fifth chapter are as follows: 1. Using the analysis of variance, the influence of the main factors on the heating efficiency was proven with the coefficient of determination of 96,2 %. The heating efficiency function exhibited the dependence on cubic polynomials of the heater temperature and on the water layer thickness. 2. A significant influence of the interaction between the water layer thickness and the heater temperature was noted. The model showed that the heating efficiency, at all levels of water layer thickness, depends on the cubic polynomial terms of the heater temperature. This means that the dependence of the heating efficiency function on the heater temperature is described by means of the cubic polynomial terms, which are different for each water layer thickness. 3. Also, simultaneous influence of the interaction between all three factors is confirmed. This means that the emission factor of the cover surface, as a categorical factor, has an influence on the interaction between the heater temperature and the water layer thickness. 4. For each category of the cover, i.e. glossy and black, a model describing the dependence of the heating efficiency on the heater temperature and the water layer thickness was established. Experimental results were then modelled by numerical simulation, which is described and its results presented in the sixth and the seventh chapter of the thesis. The results obtained by this analysis indicated the limitation of the software in performing numerical simulation, but also revealed some feasible improvements in the heating of water by radiation. The performed numerical simulation indicated the basic optical and thermal phenomena in the system that could not be detected by experimental analysis. This thesis presents a comprehensive analysis comprising theoretical considerations, the experiment, and the numerical simulation, with a goal to locate the basic phenomena influencing the process of radiation water heating and to identify their mutual influences. Identification of interrelations by which a single phenomenon affects the other/others is a first step to the optimization of a device in which the complex optical phenomena would play a major role in heat transfer.

Item Type: Thesis (Doctoral thesis)
Uncontrolled Keywords: prijenos topline zračenjem; apsorpcija vode; eksperiment; numerička analiza
Keywords (Croatian): radiative heat transfer; water absorption; experiment; numerical analysis
Subjects: TECHNICAL SCIENCE > Mechanical Engineering
Date Deposited: 22 Sep 2014 18:00
Last Modified: 10 May 2020 19:04
URI: http://repozitorij.fsb.hr/id/eprint/2100

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