26-12-2012, 03:10 PM
HEAT TRANSFER AND PRESSURE DROP AT SUPERCRITICAL PRESSURES
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ABSTRACT
Water at supercritical pressures is being increasingly used in modern power plants for
increasing the overall efficiency. The heat transfer coefficient is reported to behave
differently depending upon various parameters viz. heat flux, pressure, flow orientation,
inlet condition, mass flux etc. Numerous experimental and numerical works have been
carried out to find the variation of heat transfer coefficient and hydraulic resistance of
supercritical flow. Most of the works have been done upon round tubes both vertical and
horizontal. Correlations given by the several researchers are not consistent. This project
aims to conduct experimental investigation to study the variation of heat transfer
coefficient and hydraulic resistance at supercritical pressures and arrive at satisfactory
correlations.
Water has a critical pressure as 221 bar and critical temperature as 374.15oC. Since these
values are very high, the possibility of using alternative fluid that could have similar heat
transfer behavior was studied. Freon (R123) was selected as the modeling fluid for
further experimentation. Various components of the experimental facility were finalized
and designed. The experimental set up was built and commissioned. Several experiments
were conducted, obtained data were analyzed and results were discussed. Extensive
experimentation could not be performed due to paucity of time.
INTRODUCTION
Overview
A supercritical fluid is a fluid at pressures and temperatures that are higher than the
thermodynamic critical values. The critical point is the point where the distinction
between liquid and vapor regions disappears. Fluids at supercritical pressure have found a
large numbers of applications in recent periods. At supercritical pressures, there is no
liquid–vapor phase transition and therefore, critical heat flux or dry out does not occur.
Deterioration in heat transfer may occur only within a narrow range of parameters and
since this deterioration is gradual it does not result in the dramatic drop in heat transfer
associated with dry out. Several concepts of nuclear reactors cooled with water at
supercritical pressure to extend the performance and efficiency of the nuclear reactors are
being investigated. Using supercritical water in fossil-fired power plant is the largest
industrial application of fluids at supercritical pressures.
However, all thermo physical properties of pure fluid exhibit rapid variations at pseudo
critical point (point near to critical region, where Cp is maximum). Heat transfer at critical
and supercritical pressures is influenced by significant changes in thermophysical
properties. The heat transfer performance also fluctuates close to this region; hence, it
necessitates detailed study of heat transfer and pressure drop at supercritical pressures.
Thermophysical Properties of Fluid near Critical/Supercritical
Points
At pressure higher then critical pressure, the pseudocritical temperature is defined as the
temperature at which specific heat is maximum. General trends of various properties near
the critical and pseudocritical points are similar for many fluids. The thermophysical
properties of water and many other fluids at different pressure and temperature, including
the supercritical region, can be calculated using thermodynamic formulations. However,
data in the present case have been obtained from online database of NIST. Fig. 1.1 below
shows main thermo physical properties of water at pseudo critical (p = 25MPa) pressure.
Effect of high pressure
In accordance with the Second Law of Thermodynamics, the Rankine cycle efficiency
must be less than the efficiency of a Carnot engine operating between the same
temperature extremes.
when the average heat-addition temperature increases. Thus cycle efficiency may be
improved by increasing turbine inlet temperature (the pressure at which heat is added)
and decreasing the condenser pressure (and thus the condenser temperature). Specifically,
adding heat at highest possible average temperature will increase efficiency maximum.
Organisation of the report
Following this introduction i.e. Chapter (1), a detailed literature review has been
presented in Chapter (2). The chapter (3) provides information about selection and
analysis of modeling fluid. Chapter (4) deals with the designing and installation of
various components of the experimental set up. The experimental procedure for
conducting the experiments, obtained results are presented and discussed in Chapter (5).
The data is compared with the existing literature wherever possible; new correlations are
presented for the range of parameters where the data does not currently exist in the
literature. Chapter (6) summarizes the conclusions drawn from the present investigation
and the scope for the future work. Appendices are also provided at the end to supplement
the information.
Venturimeter
The maximum mass flux at which the experiments were planned was 1500 kg/m2-s.
Hence, a venturimeter was designed for a flow 10% in excess of the maximum mass flux
required. Finally, range of mass flux for venturimeter design was kept as 300-1650
kg/m2-s (0.024-0.13 kg/s) with operating pressure as 40 bar. The velocity at the throat
was calculated to be 2.645 m/s for the maximum flow rate and considered acceptable.
The design details of venturimeter are shown in Appendix-VIII.
Instrumentation
The instruments used to measure various process parameters as well as the safety devices
used to ensure the safety of the facility are discussed here. Fig. 4.8 shows the schematic
of the loop with the instrumentation. The instrumentation provided in the loop for
measuring the process parameters such as flow, system pressure, primary fluid
temperature and pressure drops etc is discussed in this section.
Measurement of Flow
The flow rate in the loop was measured using a venturimeter which was calibrated to
measure the flow in the range 0.024 - 0.13 kg/s, i.e. a mass flux of 300 -1500 kg/m2-s. A
calibrated Differential Pressure Transducer (DPT) in the range 0.1 to 6 KPa has been
used to measure the pressure drop across the venturimeter. The same DPT has also been
connected to measure pressure drop across the test section as shown Fig.4.8. The
calibration details of the DPT and the venturimeter are given in Appendix-IX.
Conclusions
Design, manufacturing and installation of various components of the experimental
facility were completed. The set up was equipped with necessary instrumentation for
measuring process parameters and obtaining results. Safety devices were also provided
wherever found necessary. Initial experiments were done with water to validate the set
up. Energy balance was carried out. Finally, experiments were conducted with Freon
R123 as decided. It is difficult to obtain appreciable mass flow rate in the natural
circulation loop using low heat flux. Wall temperatures along the length of the heater test
section were not in agreement with the computed values. However, obtained data are
supported by results of some of the previous work. Hence, extensive experimentation is
required to verify the data and arrive at conclusions.