Development of enhanced heat transfer components and heat pump systems for decarbonization

Patel, Purav

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https://hdl.handle.net/2142/130126 Copy

Description

Title

Development of enhanced heat transfer components and heat pump systems for decarbonization

Author(s)

Patel, Purav

Issue Date

2025-06-23

Director of Research (if dissertation) or Advisor (if thesis)

Miljkovic, Nenad

Doctoral Committee Chair(s)

Miljkovic, Nenad

Committee Member(s)

• Wang, Sophie
• Wang, Pingfeng
• He, Jiajun

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Heat pump
• Decarbonization
• Energy Storage
• Water Heating
• Two-Phase
• Brazed Plate Heat Exchangers
• Double-Wall
• Performance Enhancement
• Etching

Abstract

Heat pump systems are seen as possible solutions to achieving the societal decarbonization goals as they offer at least twice as much energy efficiency as compared to methods like electric resistance heating. However, the vapor compression heat pump systems are often restricted to operate within a range of heat source and heat sink temperature to avoid the high compressor pressure ratio and discharge temperature. Exceeding the operating temperature range can cause issues like degradation of lubricants, mechanical wear in the compressor, lower heating capacity and energy efficiency. State of the art heat pump systems aimed at improving the operating range typically employ two-stage compression with complex system architecture or rely on back up electric heating. This work introduces an alternate storage heat pump concept to improve the operating temperature range of an R134a heat pump water heater (HPWH) system. The developed system operates in two modes: Mode I functions as a typical wrap-around condenser HPWH, while Mode II splits the wrap-around coil to use the top portion as a condenser and the bottom portion as evaporator. System model consisting of vapor compression cycle and water tank models is built to predict the effect of design parameters and surrounding conditions on the system performance. Experimental analysis is carried out by modifying a conventional residential heat pump water heater unit. System performance is compared to that of a conventional heat pump system in heat pump only mode as well as with a system using electric heating elements. Results demonstrate that the storage heat pump system could achieve an additional increase of 12℃ beyond the maximum possible water temperature of 46℃ which was achieved using the baseline conventional HPWH system. In Mode II, the system exhibits a 20% higher average heating capacity, and a 15% lower average pressure ratio compared to the conventional system operating at a 27℃ ambient temperature. Compared to usage of electric heating elements, our system showed a longer recovery time with a 50% lower energy consumption for a similar increase in water temperature. Applications like potable water heating using heat pumps can also benefit from the adoption of safe and efficient heat exchangers. In this regard, the second section of this dissertation focusses on performance characterization and heat transfer enhancement for brazed plate heat exchanger (BPHX). The brazed plate heat exchanger is one of the most efficient and compact heat exchanger types used commonly as condenser and evaporator in heat pumps and chiller systems. However, the thin plates used in BPHXs are susceptible to failure caused by corrosion. To address this issue, double wall type BPHXs are employed whenever the mixing between the two fluids is to be strictly prevented. The design of double-wall brazed plate heat exchangers has not been covered much in the past and scientific literature on the topic is very limited. To bridge this gap, this work presents an experiment study on the performance characterization of double wall BPHXs in single phase heat transfer. The increased thermal resistance caused by the double wall feature is quantified in different flow conditions. The overall heat transfer coefficient in the double wall heat exchanger (double wall construction without fluid vent) is found to be same as the single wall heat exchanger at lower flow Reynolds number (Re<200) while showing about 16% lower overall heat transfer coefficient at higher flow Reynolds number (Re~800). This work also explores the potential of using chemical etching as technique to enhance the two phase heat transfer in brazed plate heat exchangers. Etching the BPHX alters the surface properties like nucleation site density and wettability by the creation of microstructures on the stainless steel plate surface. First, a suitable etching recipe for the BPHX is determined for generating microscale roughness on thin stainless steel plates. Then the performance of etched heat exchanger in two-phase heat transfer (refrigerant-to-water) is investigated and compared with a baseline unetched heat exchanger. The condensation and evaporation heat transfer of refrigerant R134a in the etched heat exchanger are examined at varying mass flux and vapor qualities. Etched heat exchanger showed only a slight improvement in the condensation heat transfer coefficient while showing about 54% and 48% higher evaporation heat transfer coefficient than the unetched heat exchanger at refrigerant mass fluxes of 18 and 44 kg/m2-s respectively.

Graduation Semester

2025-08

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/130126

Copyright and License Information

Copyright 2025 Purav Patel

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