Adiabatic heat development, 2.18

In the work specifications for aU major concrete work it is specified that temperature differences in the hardening concrete must be kept within certain limits. Therefore, it is in many cases necessary to make advance calculations of the temperature development in the hardening cross-sections under different conditions, and then, based on this, decide upon a method of execution that safeguards the concrete against crack damages during hardening. In snch calcnlations the data of the adiabatic heat development of the applied concrete are used. 

Calculate and draw a graph showing the adiabatic heat development Q as a function of time t the heat development Q shonld be given in kJ per kg of cement. 

With the expression (c) the measured temperature rise in the specimen can be converted to adiabatic heat development... 

Calculation of heat capacity and adiabatic heat development (15 min.)... 

Discussion. Experimental determination of the adiabatic heat development can, for example, be performed with an adiabatic calorimeter, as shown in figure 2.29. For casting and hardening of small units under normal, i.e. non-adiabatic, conditions the temperature rise is limited by the heat loss during hardening. 

The micro-channels utilized in engineering systems are frequently connected with inlet and outlet manifolds. In this case the thermal boundary condition at the inlet and outlet of the tube is not adiabatic. Heat transfer in a micro-tube under these conditions was studied by Hetsroni et al. (2004). They measured heat transfer to water flowing in a pipe of inner diameter 1.07 mm, outer diameter 1.5 mm, and 0.600 m in length, as shown in Fig. 4.2b. The pipe was divided into two sections. The development section of Lj = 0.245 m was used to obtain fully developed flow and thermal fields. The test section proper, of heating length Lh = 0.335 m, was used for collecting the experimental data. 

Adiabatic Ignition of Propellants, Pyrotechnic Compositions, etc. When ign of a subst is effected in a highly insulated condition with no gain of heat from or loss of heat to the system, it is called adiabatic ignition. The ign can be initiated by a spark, flame, incandescent wire, etc and the heat developed by these sources must be taken into consideration when calculating the heat of expin or deton from experimental data... 

The theory of adiabatic reaction developed in the previous article is here generalized to the case when heat transfer is present. Consideration of the heat transfer leads to the appearance of new features in the consumptiontime kinetic curves, specifically, the possibility of extinction as the residence time is increased and of self-ignition when the reaction time is decreased (in the previous article, in the adiabatic case, extinction occurred only for a decrease in the reaction time, and self-ignition only for an increase). 

With regard to application and construction, it is convenient to differentiate between fixed-bed reactors for adiabatic operation and those for nonadiabatic operation. Since temperature control is one of the most important methods to influence a chemical reaction, adiabatic reactors are used only where the heat of reaction is small, or where there is only one major reaction pathway in these cases no adverse effects on selectivity or yield due to the adiabatic temperature development are expected. The characteristic feature of an adiabatic reactor is that the catalyst is present in the form of a uniform fixed bed that is surrounded by an outer insulating jacket (Fig. 1A). Adiabatic reactor designs are discussed in Section 10.1.3.1. 

Boreskov had a sharp sense of the new and always supported the appearance of novel trends in catalysis. He directed experiments on the application of catalysis for fuel combustion and participated in the development of new methods to carry out catalytic processes—performance of reactions in unsteady state conditions (a promising way of performing exothermal reactions with low adiabatic heat). 

Industrial reactors used in the petrochemical industry for exothermic reactions, with a few exceptions, are either fixed-beds (adiabatic or non-adiabatic) or fluidized-beds when the heat developed is too high to be removed in a fixed-bed reactor. In the last few decades, interest has been mainly directed towards the control of these reactors, which is strictly related to an understanding of the complex phenomena that occur at the interface between the different phases present in the reaction environment, and of the heat and mass transfer influence on the reaction kinetics. 

Concrete consists of a mixture of cement, water and coarse and fine aggregates. During the reaction between the cement and the added water, the concrete hardens and gains strength. The chemical reactions between cement and water - the so-called hydration - are marked by considerable heat development. If concrete hardens under adiabatic conditions, i.e. without exchanging heat with its surroundings, the concrete temperature will typically increase by 50-70 °C due to the heat developed. 

An adiabatic caiorimeter is used for measuring heat development of concrete under adiabatic hardening conditions. Samples of fresh concrete are used for the measurement. Heat exchange with its snrronndings is prevented during the hardening of the concrete, so that the heat developed is fuUy converted to a temperature rise in the test sample. The temperature rise is measured as a function of time. Finally, with knowledge of the heat capacity and cement content of the sample, the quantity of heat Q (in kJ per kg of cement) can be calcnlated. 

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