الفهرس 
Ejector Expansion Refrigeration CycleOnly 14 pages are availabe for public view
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Abstract
Ejectors are used in several engineering applications, such as steam power
plants, cooling of nuclear systems, mixing processes and refrigeration systems,
because they have many advantages over conventional compression systems.
These include no moving parts and hence no lubrication is needed. The relatively
low capital cost, simplicity of operation, reliability and very low maintenance cost
are other advantages. The major drawback is the low efficiency. The ejector,
employed as a fluid pumping device, represents a well-known technology for
industrial refrigeration in order to reduce the required compressor work. Therefore,
the objective of the present work is to describe and evaluate a reliable numerical
procedure for designing two-phase (gas-liquid) ejectors. The method predicts
numerically the optimum geometry of an ejector which gives maximum efficiency.
The numerical investigation is based on non-homogeneous (liquid and
vapor velocities are not equal), non-equilibrium (liquid and vapor temperatures are
not equal), two-fluid Eulerian-Eulerian model (both liquid and gas phases are
considered as separate fluids), conservation equations governing steady, two–
dimensional (axisymmetric), turbulent, compressible, and parabolic two-phase
flow. These equations are namely continuity, momentum, and energy. These
equations are discretized using finite volume method and solved iteratively based
on the SIMPLE algorithm, [87]. Turbulence model used is Prandtl’s mixing length.
In addition, interfacial momentum, mass and heat transfer between the liquid and
vapor phases are considered.
Furthermore, wall function is used instead of using very fine grid near the
wall. The coordinates system is converted to bodyfitted coordinates. Refrigerant
134a is used as a working fluid. The Modified Benedict-Webb-Rubin (MBWR),
[82] equation of state is used to represent compressibility. The presented model is
validated against previously published data in literature. The validation showed
reasonable agreement.
In the present Ph.D. thesis, the theoretical results are concerned with
investigating, separately in details, the effects of changing both geometrical and operational parameters on two-phase flow ejector performance. The operational
parameters include stagnation pressure coefficient, mass flow ratio and motive
flow inlet steam quality and on the contrary, the geometrical parameters include
motive flow nozzle area ratio, ejector area ratio, convergent mixing section length,
constant-area mixing section length, diffuser section length, convergent and
divergent section total angles. The theoretical results obtained help to understand
the two-phase flow behavior and physical phenomena occurring during mixing
process of two-phase flow through ejectors. On the other hand, the results are used
as well to develop optimum ejector design charts and correlate the optimum ejector
dimensionless geometrical parameters, ejector compression ratio, primary flow
nozzle expansion ratio as functions of the mass flow ratio.
It is concluded that, for maximum ejector efficiency (optimum design) both
geometrical and operational parameters must be carefully selected through the
resultant state-of-an-art correlations.
Overall, the results lead to useful information for ejector optimum design
and prediction of the two-phase flow ejector behavior and performance.