Miroslav Příhoda

SIMULATION OF HEAT TRANSPORT AT THE COOLING OF THE SUGAR SOLUTION IN A RECUPERATION EXCHANGER

The paper describes a mathematical model of the cooling process of a highly concentrated sugar solution in an exchanger with a specifically shaped heat exchanging surface of the cooling panels. An analysis of the individual parts of the stum cooling line is made, dealing with the cooling performance of the cooling panels located in the stum tanks, whose volume is 3230 litres or 1430 litres. One of the monitored parameters is the cooling performance of the JN30 aggregate. The article also deals with an appropriateness of the aggregate for cooling the stum of the total volume 78.21 m 3 , from the real operation temperature to 0 °C during 48 hours.

Heat output of the recuperator for preheating natural gas from a mobile container device

The paper describes the methodology for determination of the heat exchanger output for mobile container device designed to supply stationary stations with natural gas. Expansion of the gas in the device is considered in two stages. In the first stage from over-pressure of 22 MPa down to over-pressure of 0.4 MPa and then in the second stage to over-pressure of 2.2 kPa or 170 kPa. The overall temperature drop during gas throttling, which depends not only on the initial and final pressure but also on the temperature, was calculated by numerical integration of the Joule-Thomson differential coefficient. For the throttling temperature between 0 and 55 °C, the total temperature drop in the first stage varies between 53.8 and 67.4 K, and in the second stage then up to 2 K. The heat exchanger output was determined by an iterative process. Smooth supply of 600 m3 of natural gas per hour requires a heat exchanger with an output of approx. 26 kW. Gas combustion micro-turbine consumption is of the order of m3 of natu...

APPLICATION OF DIMENSIONAL ANALYSIS IN HEAT LOSS DETERMINING IN DISTRICT HEATING SYSTEMS

Existing procedures for determining the heat loss in heat distribution use balance relations, or rather they are based on the theory of heat and mass transfer. Dimensional analysis enables a new point of view that demonstrated the functionality of heat loss from selected physical quantities that contribute to heat loss. The solution provides one complex criterion and four simplex criteria of similarity. Transforming these criteria of similarity leads to the formulation of the functional dependence of only two criteria, on the basis of which the amount of heat loss can be determined. The resulting criterial dependency is simple, and is calculated in this paper for a pipe one meter in longth. The mathematic model for demonstrating the heat loss is of universal validity, and applies to a wide range of piping used for hot water distribution. However, for each nominal diameter of piping it is necessary to take into account the mutual dependency of the dimensionless arguments p 5 on p 1 , the form of which is always different. In the paper, this dependency is demonstrated for two nominal diameters DN65 and DN125.

Power losses and the efficiency of industrially used gearboxes

This article gives the methodology for stating power losses and the efficiency of a gearbox working in natural cooling using dimensional analysis. The methodology reflects experimental research which investigated the effect of a change in revolutions and loading the gearbox on power losses. At the same time, it describes the conditions under which the proposed method may be applied for industrial, helical bevel geared motors.

Monitoring and simulation of the unsteady states in continuous casting

Kontinuirno litje obsega termicne, mehanske in kemicne procese, ki tecejo v kompleksnem sistemu, ki vsebuje vrsto elementov, kot so: strjujoca se jeklena žila (gredica), kokila z oscilacijskim mehanizmom, izvlecni mehanizem, vodno hlajenje s podsistemom hladilnih sob, vec kontrolnih podsistemov itd. Zunanji opazovalec lahko vidi proces kot robusten in stabilen, toda v resnici imamo vrsto fluktuacij (nihanj) internih termicnih in mehanskih velicin in kakovosti produkta (nastajajoce konti lite gredice). Raziskave (casovno) nestabilnega obnasanja velicin, kot so: temperaturno polje strjujoce se konti gredice, debelina trdne skorje in metalurska dolžina, so avtorji prispevka izvajali z industrijskim diagnosticnim sistemom (DGS), dopolnjenim s specialno merilno opremo in termicnim numericnim modelom. V clanku avtorji predstavljajo izbrane rezultate analiz, opazovanja in simulacije nestandardni procesnih stanj. Prav tako predstavljajo metode dolocevanja robnih pogojev za numericni model. Diskusija obsega tudi t.i. Leidenfrostov fenomen in njegov vpliv na koeficient prenosa toplote med vodnim hlajenjem s sobami. Za natancno dolocitev vsakokratnih robnih pogojev obstajajo tehnicne omejitve. Zato so v postavljenem modelu uporabljene le zglajene vrednosti v realnem casu in prostoru. Poznavanje trenutnega stanja ves cas spreminjajocega stanja, je predpogoj za doseganje kvalitetne proizvodnje brez napak. Zato je primerno uporabljati termicni numericni model s tekocim (on-line) oz. neposrednim spremljanjem internega stanja na konti livni napravi. Predstavljeni rezultati so tesno povezani z realnimi procesnimi podatki.

High-temperature processing and recovery of autocatalysts

V tem clanku avtorji predstavljajo rezultate eksperimentalnih raziskav termicnega postopka pridobivanja kovin platinske skupine (PGM, angl.: Platinum Group Metals) s taljenjem izrabljenih avtomobilskih katalizatorjev v 80 kVA plazemskem reaktorju z uporabo primernih talil in redukcijskega sredstva. Kot zbiralnik plemenitih kovin je bila med taljenjem uporabljena siva litina. Produkti taljenja so zlitina, inertna steklasta žlindra, sintezni plin in pepel. V zlitini je bilo pri taljenju katalizatorja na osnovi kordierita koncentriranih 99,27 % PGM, pri katalizatorjih s Cr/Ni trakom pa 99,00%. Pri taljenju kordieritnih katalizatorjev se je v žlindri izgubilo 0,34 % PGM, pri Cr/Ni katalizatorjih pa 0,26 %. Kemijska analiza pepela je potrdila, da poleg mehansko pridobljenih oksidov CaO, SiO2, MgO, Al2O3, le-ta vsebuje tudi kondenzirane pare Fe in PGM. V pepelu se je pri taljenju katalizatorja na osnovi kordierita nahajalo 0,40 % plemenitih kovin, pri katalizatorju s Cr/Ni trakom pa 0,74 %. Sintezni plin iz procesa taljenja je imel zelo nizko kurilno (kaloricno) vrednost (0,16 MJ m–3 oz. 0,24 MJ m–3).

Pyrometallurgical treatment of silver-containing catalysts

The following article describes the thermal process for the recovery of used silver from Ag catalysts using an 80-kVA plasma reactor, along with an appropriate flux and reducing agent. An Ag catalyst was melted in three separate experiments with different weights. The overall recovery of silver from the melted Ag catalysts was high (93.8–96.4) %. The byproducts of the melting of the Ag catalysts were the inert vitrified slag, synthesis gas and fly ash. The chemical analysis of fly ash confirmed that, in addition to mechanically stripped oxides that were present in the batch (CaO, SiO2, MgO, and Al2O3), fly ash also contained a high amount of condensed silver. Indeed, silver evaporated at a high temperature during the melting process. The silver condensed in fly ash was at a level of 37.08–48.37 % of the total weight of fly ash. Therefore, fly ash had to be recycled. The synthesis gas from the process had a relatively low heating value (0.6335 MJ m).

Model of heat losses from underground heat distribution system

The paper describes the linear density of the heat flow using dimensional analysis in the case of underground, pre-insulated piping. The methodology makes it possible to determine the heat loss based on a simple criterial relationship. Heat losses in the piping system were also calculated using numerical simulation. The difference between the experimentally detected losses and the values obtained from the model is −3.6% to 6.4%. The results of the numerical simulation at an ambient temperature of 25°C differ from the experiment by −1.7%. At an ambient temperature of −10°C the difference is −4.0%.The paper describes the linear density of the heat flow using dimensional analysis in the case of underground, pre-insulated piping. The methodology makes it possible to determine the heat loss based on a simple criterial relationship. Heat losses in the piping system were also calculated using numerical simulation. The difference between the experimentally detected losses and the values obtained from the model is −3.6% to 6.4%. The results of the numerical simulation at an ambient temperature of 25°C differ from the experiment by −1.7%. At an ambient temperature of −10°C the difference is −4.0%.

Monitoring of solidification in the continuous casting mold by temperature sensors

Defects of continuously cast strand, such as unevenness of shell thickness or cracks as well as unstable casting parameters result in changes of strand surface temperature which affect heat flux and temperature field in the mold wall. Methods based on the principle of measurement and mathematical processing of temperatures in the mold wall are used for the purposes of diagnostics of the shell formation process, prediction of surface and subsurface quality and breakout danger, adjustment of the casting axis or condition monitoring of the oscillating mechanism. Measured values of temperatures in the wall depend on the exact position of the sensor in the wall, especially in the normal direction to the mold working surface. Ensuring the accurate and constant distance between the sensor and the mold surface is technically demanding; therefore it is necessary to correct the measured temperatures mathematically. The article describes two methods for correcting the measured temperatures, based on physical and sta...

Analysis of the Subsonic and Supersonic Flow Using Analytical and Numerical Methods

The article presents the analysis of the 1D flow of compressible fluid by means of analytical and numerical methods. The results from the solution showed that the calculation of dimensionless velocity for particular flow conditions varies in the area of subsonic flow only a very little, when using both methods. It was found that the dependence of dimensionless velocity on the relative duration of the investigated tunnel applies universally. For any proportional value of the tunnel length x/L and the constant ratio of outlet and inlet cross section of the tunnel level equal to 0.6474, the course of the dimensionless velocity for each tunnel, which satisfies the above condition, will always be the same. This means that also the nature of flow in any such tunnel will exhibit the same properties. This finding provides new knowledge from the analysis of air flow through a channel with a variable flow cross section.