Cycle-averaged temperature

The steady-state cycle-averaged temperature is governed by the Laplace equation ... 

As has been mentioned in Sect. 7.3, the continuous injection-molding operation results in a cyclic heat transfer behavior in the mold, after a short transient period. The cycle-averaged temperature can be represented by a steady state heat conduction equation, i.e., Eq. 7.10. The mold cooling analysis can be greatly simplihed by solving the steady state problem. The boundary integral equation of ( 7.10) is... 

COMMENTS (1) The sensitivity diagram of cycle efficiency versus boiler pressure demonstrates that increasing the boiler pressure increases the boiler temperature. This raises the average temperature at which heat is added to the steam and thus raises the cycle efficiency. Operating pressures of boilers have increased over the years up to 30 MPa (4500psia) today. 

Decreasing the average temperature during the heat-rejection process decreases the condenser pressure and increases cycle efficiency. The minimum condenser pressure is limited by the sealing and leakage problem in the condenser. 

Increasing the average temperature during the heat-addition process with a reheater without increasing the compressor pressure ratio increases the net work of the Brayton cycle. A multistage turbine is used. Gas is reheated between stages. 

The recuperated Brayton cycle approaches Carnot efficiency in the ideal limit. As compressor and turbine work are reduced, the average temperatures for heat addition and rejection approach the cycle limit temperature. The limit is reached as compressor and turbine work (and cycle pressure ratio) approach zero and fluid mass flow per unit power output approaches infinity. It can be expected from this that practical recuperated Brayton cycles would operate at relatively low pressure ratios, but be very sensitive to pressure drop. With tire assumption of constant gas specific heat over the cycle temperature range, a good assumption for helium, the cycle efficiency of a recuperated Brayton cycle may be expressed ... 

The carbon cycle has continued in this manner for millions of years. However, scientists have detected an imbalance in the carbon cycle due to the increase in the amount of carbon dioxide produced through burning fossil fuels and the deforestation of large areas of tropical rain forest. The Earth s climate is affected by the levels of carbon dioxide (and water vapour) in the atmosphere. If the amount of carbon dioxide, in particular, builds up in the air, it is thought that the average temperature of the Earth will rise. This effect is known as the greenhouse effect (Figure 13.14). 

Climate researchers Palutikof and Holt (2004) say that droughts appear to be linked to the formation of blocking zones of intense high pressure over the Atlantic Ocean that divert rain-bearing wind depressions away from the Mediterranean. This blocking may be related to the cycles of El Nino, the periodic reversal of winds and waves in the tropical Pacific Ocean. Due to the effects of toxic gases, it is assumed that year-round average temperature in southern Europe will reach 18°C by the year 2030 and rainfall will diminish by 19% to 20%. 

Using an ideal cycle and available levels of temperatures Tm, Th and Th it is possible to estimate theoretical maximal differences of temperatures of heating and cooling of hydride from average temperature with reference to problems of a heat transformation and production of a cold ... 

From formulas it is visible, that at obtaining of additional heat and condition ATh=AT it is necessary to have lower value A7), rather than, than at obtaining of a cold value AT,. It is connected to value of an additional factor. Substituting in these formulas the initial data on temperature of hydrides, it is possible to find out the maximal possible temperature deviations in a cycle from average temperature and to estimate an opportunity of performance of a mission of HHP (obtaining of a heat/cold of the necessary potential). 

This type of periodic operation allows for conversion improvement in reversible exothermic reactions [9], A cycle average inlet temperature for the conditions of continuous temperature oscillation can be substantially lower than the inlet temperature under steady-state conditions. This leads to a lower outlet temperature and higher equilibrium conversion for a reversible reaction. Better performance is achieved if temperature oscillations attenuate sufficiently during the passage through the catalyst bed [9]. 

The purpose of heating the feedwater in tliis manner is to raise the average temperature at wliich heat is added in the boiler. This increases the tliennal efficiency of tire plant, wliich is said to operate on a regenerative cycle. 

Feedwater heaters increase plaint efficiency by increasing the average temperature at which heat is supplied to the cycle H2O from the products of combustion (7, articles 15.1-6) that is, inasmuch as the available-energy consumption for driving heat transfer processes increases with the temperature difference... 

Figure 6.17. Observations of the average temperatures at 65° S and 65° N from the Fleming et al. (1990) Cospar International Reference Atmosphere (CIRA) climatology (top). The middle and bottom panels show satellite measurements of the annual cycles of total ozone at 65°N and 65°S in the late 1970s and in 1996 and 1997 (from SBUV/SBUV2, courtesy of R. Nagatani). From Solomon (1999).

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