A counter-flow heat exchanger transient one-dimensional numerical model was developed and verified against other authors results. Heat exchangers have been extensively studied due to their vast number of industrial applications where steady state operation is usually assumed. However, heat exchangers are also used in systems that operate under transient conditions such as energy generation, energy conversion, or energy storage.

After a detailed literature review, it was determined that there was a need for a more comprehensive study on the transient behavior of heat exchangers. Computational power was not readily available when most of the work on transient heat exchangers was done (1956 – 1986), so most of these solutions have restrictions, or very specific assumptions. More recently, authors have obtained numerical solutions for more general problems (2003 - 2013), but they have investigated very specific conditions, and cases. For a more complex heat exchanger (i.e. with heat generation), the transient solutions from literature are no longer valid. There was a need to develop a numerical model that relaxes the restrictions of current solutions to explore conditions that have not been explored.

A one dimensional transient heat exchanger model was developed. There are no restrictions on the fluids and wall conditions. The model is able to obtain a numerical solution for a wide range of fluid properties and mass flow rates. Another innovative characteristic of the numerical model is that the boundary and initial conditions are not limited to constant values. The boundary conditions can be a function of time (i.e. sinusoidal signal), and the initial conditions can be a function of position. Four different cases were explored in this work. In the first case, the start-up of a system was investigated where the whole system is assumed to be at the same temperature. In the second case, the new steady state in case one gets disrupted by a smaller inlet temperature step change. In the third case, the new steady state in case one gets disrupted by a step change in one of the mass flow rates. The response of these three cases show that there are different transient behaviors, and they depend on the conditions imposed on the system. The fourth case is a system that has a sinusoidal time varying inlet temperature for one of the flows. The results show that the sinusoidal behavior at the inlet propagates along the channel. However, the sinusoidal behavior on one of the fluids does not fully translate to the other gets damped by the wall and the heat transfer coefficients that can be barely seen on the other flow.

A scaling analysis and a parametric study were performed to determine the influence the different parameters on the system have on the time a heat exchanger takes to reach steady state. The results show the dependency of tst* (time a system takes to reach steady state) on the dimensionless parameters M, C, NTUh, NTUc, and Cw. tst* depends linearly on C and Cw, and it is a power function of M. It was also shown that tst* has a logarithmic dependency on NTUh and NTUc. A correlation was generated to approximate the time a system takes to reach steady state for systems where Cw << 1.

A more complex heat exchanger with the specific application of solar energy storage was also investigated. This application involves a counter-flow heat exchanger with a reacting flow in one of the channels, and it includes varying properties, heat generation, varying heat transfer coefficient, and axial conduction. The application for this reactor heat exchanger is on solar energy storage, and the goals is to heat up steam to 650 °C by using the ammonia synthesis heat of reaction. One of the concerns for this system is the start-up time and also how disturbances in reacting flow can affect the steam outlet temperature. The transient behavior during the system start-up was presented. In order to achieve the desired outlet steam temperature at a reasonable time, the system must operate at high gas mass flow rates. If the inlet temperature of the gas suffers a step change, it affects the reaction rate as well as the outlet steam temperature. A small perturbation on the gas mass flow rate has an effect on the profile shape. However, the maximum temperature reached by the gas due to reaction is not affected, and consequently, it has little effect on the steam temperature.

Axial conduction in the reactor heat exchanger was also investigated, specifically in the gas section. Axial conduction cannot be assumed to be negligible in the reactor heat exchanger because of the iron-based catalytic bed. Results in this section show that axial conduction is detrimental for the system. It was found that for Peclet number greater than 100, axial conduction can be neglected. An alternative solution to address axial conduction was proposed, namely to include a well-insulated non-reacting section (without a catalytic bed) upstream of the reactor. The modified reactor heat exchanger was a novel solution to avoid the negative effect of axial conduction. Results show that by having a non-reacting section, axial conduction becomes unimportant.

An anaerobic digester is a part of a wastewater treatment technology and its role is to digest the wastewaters to treat biosolids. Anaerobic digesters typically operates at either 35�C to 37�C or 45�C, and biosolids must be heated from ambient temperature (10-20�C). To heat the sludge in anaerobic digesters steam injection is used which dilute the sludge and the requirement of electricity to heat steam is not economically favorable. Conventional heat exchangers have been used in wastewater treatment plants (WWTP) but there are many difficulties regarding the maintenance, clogging and cleaning, which discourages their use, especially for heat recovery.

Methane gas (CH4) is produced in digesters which is combusted and used for heating the buildings. In the case of large WWTPs, combusted methane is also used for power generation; but it usually combusted to the air. There are largely unused opportunities for heat recovery and it is not practiced due to the low potential heat recovery as well as maintenance problems with heat exchangers.

Heat pipes are an interesting technology and have not been used in WWTPs. In this dissertation heat pipes for wastewater treatment (low temperature heat recovery) and combusted biogas products’ applications are evaluated. Then heat pipe heat exchangers (HPHE) is designed and the simulations of heat pipe heat exchangers (HPHE) for different fluid pairs at Cr<<1 and Cr≈1 is demonstrated and then compared with a conventional heat exchanger (spiral heat exchanger). Then a method to justify when using HPHE is favorable for any design and application is introduced.

Transient thermal system management is promising in various engineering disciplines such as thermal management of electronics [CKK98] and aerospace systems [DYB17]. There is a need for an approach to estimate the system’s complex status to exert control and perform design optimization. Such an approach should have a low calculation cost while capturing the essential flow physics such as the phase change heat transfer and nucleate boiling. As a representative of transient thermal systems, Oscillating Heat Pipes (OHPs) consist of a serpentine capillary channel partially filled with liquid that is embedded in a thermally-conducting solid. The model reported here aims to capture the essential physics of an OHP with minimal complexity and treats some parameters typically derived from correlations or experiments (such as the film thickness and film triple point velocity) as functions with tunable constants to be estimated by data assimilation. This model contains two modules. The first uses a novel and flexible formulation of the conducting solid, solving the two-dimensional heat equation in a thin plate, with evaporators and condensers as immersed forcing terms and the OHP channel as an immersed line source. The second module solves one-dimensional fluid motion and heat transfer equations within the fluid-filled channels based on mass, momentum, and energy conservation, nucleate boiling, and bubble dryout. It extends the commonly-used film evaporation-condensation model, allowing both variable liquid film thickness and length and thereby enabling the model to capture dryout. These modules are weakly coupled, in that wall temperature in the channels is obtained from the first module and heat flux from the channels determines the line source strength. After minimal training, the thermal conductance calculated by this model shows good agreement with a wide range of experiments performed by Drolen et al. [DWT22]. In particular, the model successfully predicts the experimentally-observed transition from stable OHP operation to dryout, for the first time to the authors’ knowledge. Then the model is used to study the gravity effects of a ground-tested OHP and explored possible reasons of the discrepancy between the simulation and the ground experiments. This thesis also shows some data-assimilation techniques performed, such as using ensemble-Kalman Filter to estimate condenser contact condutance using a simple model adapted from Zhang et al. [ZFS02] and using Markov Chain Monte-Carlo method to sample inertial thickening parameters and plate thickness using the proposed model.