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
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
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 (english)|| |
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