Liquid fuel is evaporated and mixed with an oxidizing agent in a mixing chamber of a reformer by first introducing substantially only the liquid fuel in an axial downstream direction into a first upstream zone of the mixing chamber via a nozzle so as to atomize the liquid fuel. Then steam is separately introduced into the upstream zone and contacted with the atomized fuel to cause the fuel to evaporate. An oxidizing agent is then introduced downstream of the first zone in a second zone of the mixing chamber to the evaporated fuel and uniformly mixing the oxidizing agent with the evaporated fuel.