A Novel Generator Design Utilised for Conventional Ejector Refrigeration Systems
Abstract
:1. Introduction
2. System Description
- the heat exchanger operated under steady-state conditions and was considered as one-pass counterflow;
- heat losses to the environment are negligible;
- the maldistribution phenomenon inside the PHE was neglected;
- for the cold stream with a phase change, the plate was divided into three portions;
- a linear temperature difference was considered between the two flow streams;
- the cold stream was modelled at a saturation temperature of 87 °C and an inlet temperature of 25 °C;
- effectiveness was estimated using the ϵ-NTU method; and
- all the thermophysical properties were estimated based on the NIST fluid database (REFPROP v.10).
3. Theoretical Analysis
3.1. Data Reduction
3.2. Calculation Procedures
- The inputs are the hot stream pressure () and the superheat difference (). Afterwards, the saturation pressure () was calculated according to the refrigerant name () at 87 °C.
- The enthalpies were estimated for all artificial inlets and outlets of the cold stream.
- The total heat rate () was evaluated for each artificial section according to the enthalpy difference multiplied by the mass flow rate of the cold stream, Equation (1).
- The inlet enthalpy and the temperature of the hot stream ( and ,respectively) were estimated using the compressor model and the hot stream pressure (). Subsequently, the other enthalpies and temperatures were evaluated using the corresponding heat rate () from each artificial section.
- If the outlet temperature of the hot stream’s second section () is higher than the cold stream saturation temperature (), then the required dimensionless numbers of each stream can be evaluated directly except for the boiling number (Bo), as it requires information on the heat flux between the hot and cold streams (). Based on Amalfi’s review [22], the average between the maximum and the minimum Boiling number was taken as an initial guess with a value of 0.002.
- In the next step, the overall heat transfer coefficient (), the heat flux (), the number of transfer units (NTU), and the effectiveness () will be estimated. When all the data are calculated, the boiling number (Bo) will be updated according to the new heat flux () as well as the overall heat transfer coefficient (), the number of transfer units (NTU), and the effectiveness (). Then, the mass flow rate () will be updated.
- Alternatively, if the outlet temperature of the hot stream’s second section () is lower, then the calculation will be stopped for the current cold stream mass flow rate, and the name (Ns) will be updated to the next step.
- Finally, the output is stored as the effectiveness, the convection coefficients, and the pressure drop matrices at various channel mass fluxes.
4. Discussion
4.1. Effect of Hot Stream Inlet Pressure
4.2. Effect of Superheat
4.3. Effect of Heat Exchanger Size
5. Conclusions
- when increasing the hot stream inlet pressure from 9 to 12 MPa, the two-phase convection coefficients of the HFOs increase significantly with a range of 20–50% and 20–200% for R1234yf and R1234ze(E), respectively;
- increasing the cold stream superheat temperature difference from 5 K to 20 K allows the two-phase convection coefficients of the HFOs to increase steeply with a value of 1–20% at low cold channel flow fluxes;
- the two-phase convection coefficient is significantly influenced by the boiling and bond numbers. The working fluid type affects the bond number, while the heat flux dominates the boiling number;
- increasing the plate size influenced both the two-phase convection coefficient and the effectiveness significantly. The two-phase convection coefficients of the HFOs increased with a range of 1–20% and 1–50% for R1234yf and R1234ze(E), respectively. In contrast, the effectiveness increased by 10% for R1234yf and 5% for R1234ze(E);
- the two-phase convection coefficient dominates the frictional pressure drop as the flow is turbulent even at a low vapor quality.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
Nomenclature
Units and Symbols | |
Mass flow rate (Kg/m2) | |
A | Area (m2) |
b | Mean channel gap (m) |
Bd | Bond number (-) |
Bo | Boiling number (-) |
C | Heat capacity (KW/K) |
Cp | Specific heat (Kj/Kg.K) |
D | Diameter (m) |
f | Fanning friction coefficient (-) |
F | Fouling Factor (m/s2) |
g | Gravity acceleration (m/s2) |
G | Mass flux (Kg/m2·s) |
Gr | Grashof number (-) |
H | Enthalpy (Kj/Kg) |
I | Enthalpy of vapourization (Kj/Kg) |
h | Convection coefficient (-) |
L | Effective flow length (m) |
NTU | Number of transfer units (-) |
Nu | Nusselt number (-) |
p | Plate pitch (m) |
P | Pressure (MPa) |
PHE | Plate heat exchanger (-) |
Pr | Prandtl number (-) |
q | Heat flux (KW/ m2) |
Q | Heat rate (KW) |
Re | Reynolds number (-) |
t | Thickness (m) |
W | Plate width (m) |
Greek symbols | |
∅ | Enlargement factor (-) |
µ | Dynamic Viscosity (kg/m.s) |
β | Corrugation Angle (°) |
Δ | Difference (-) |
ε | Effectiveness (-) |
κ | Conductivity (-) |
λ | Corrugation Pitch (kg/m3) |
ρ | Density (kg/m3) |
Surface tension. (N/m) | |
Subscripts | |
* | Reduced |
a | Martin first constant |
1 | First partition |
b | Martin second constant |
2 | Second partition |
3 | Third partition |
c | Cold |
ch | Channel |
eff | Effective |
h | Hot |
hy | Hydraulic |
i | Inlet |
k | Artificial partition index |
l | Liquid |
lo | Liquid only |
lv | Liquid vapour difference |
m | Mean |
o | Outlet |
p | Plate |
s | Refrigerant type index |
sat | Saturated |
SH | Superheated |
sp | Single phase |
tp | Two phase |
v | Vapour |
vo | Vapour only |
w | Wall |
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Geometry Parameters | Plate Size | |
---|---|---|
PHE#A | PHE#B | |
Effective flow length, L (mm) | 485 | 250 |
Plate thickness t_p (mm) | 0.60 | 0.60 |
Port diameter D_p (mm) | 55 | 30 |
Corrugation angle, β (°) | 60 | 60 |
Plate Pitch, P (mm) | 2.8 | 2.50 |
Mean Channel Gap, b (mm) | 2.2 | 1.90 |
Plate width, W (mm) | 245 | 95 |
Corrugation pitch, λ (mm) | 6.80 | 6.80 |
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Elbarghthi, A.F.A.; Hdaib, M.Y.; Dvořák, V. A Novel Generator Design Utilised for Conventional Ejector Refrigeration Systems. Energies 2021, 14, 7705. https://doi.org/10.3390/en14227705
Elbarghthi AFA, Hdaib MY, Dvořák V. A Novel Generator Design Utilised for Conventional Ejector Refrigeration Systems. Energies. 2021; 14(22):7705. https://doi.org/10.3390/en14227705
Chicago/Turabian StyleElbarghthi, Anas F. A., Mohammad Yousef Hdaib, and Václav Dvořák. 2021. "A Novel Generator Design Utilised for Conventional Ejector Refrigeration Systems" Energies 14, no. 22: 7705. https://doi.org/10.3390/en14227705